%
% This file was created by the TYPO3 extension
% bib
% --- Timezone: CET
% Creation date: 2019-02-20
% Creation time: 18-30-36
% --- Number of references
% 130
%
@Article { Zukovic_Phys.Rev.Lett2018,
title = {Dynamical Quantum Phase Transitions in Spin Chains with Long-Range Interactions: Merging Different Concepts of Nonequilibrium Criticality},
journal = {Physical Review Letters},
year = {2018},
month = {3},
day = {27},
volume = {120},
pages = {130601},
abstract = {We theoretically study the dynamics of a transverse-field Ising chain with power-law decaying interactions characterized by an exponent \(\alpha\), which can be experimentally realized in ion traps. We focus on two classes of emergent dynamical critical phenomena following a quantum quench from a ferromagnetic initial state: The first one manifests in the time-averaged order parameter, which vanishes at a critical transverse field. We argue that such a transition occurs only for long-range interactions \(\alpha\)\(\le\)2. The second class corresponds to the emergence of time-periodic singularities in the return probability to the ground-state manifold which is obtained for all values of \(\alpha\) and agrees with the order parameter transition for \(\alpha\)\(\le\)2. We characterize how the two classes of nonequilibrium criticality correspond to each other and give a physical interpretation based on the symmetry of the time-evolved quantum states.},
web_url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.130601},
DOI = {10.1103/PhysRevLett.120.130601},
author = {Zunkovic, Bojan and Heyl, Markus and Knap, Michael and Silva, Alessandro}
}
@Article { Pancotti_Phys.Rev.B2018,
title = {Almost conserved operators in nearly many-body localized systems},
journal = {Physical Review B},
year = {2018},
month = {3},
day = {23},
volume = {97},
pages = {094206},
abstract = {We construct almost conserved local operators, that possess a minimal commutator with the Hamiltonian of the system, near the many-body localization transition of a one-dimensional disordered spin chain. We collect statistics of these slow operators for different support sizes and disorder strengths, both using exact diagonalization and tensor networks. Our results show that the scaling of the average of the smallest commutators with the support size is sensitive to Griffiths effects in the thermal phase and the onset of many-body localization. Furthermore, we demonstrate that the probability distributions of the commutators can be analyzed using extreme value theory and that their tails reveal the difference between diffusive and subdiffusive dynamics in the thermal phase.},
web_url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.094206},
DOI = {10.1103/PhysRevB.97.094206},
author = {Pancotti, Nicola and Knap, Michael and Huse, David A. and Cirac, J. Ignacio and Ba{\~n}uls, Mari-Carmen}
}
@Article { Bohrdt_Phys.Rev.B2018,
title = {Angle-resolved photoemission spectroscopy with quantum gas microscopes},
journal = {Physical Review B},
year = {2018},
month = {3},
day = {13},
volume = {97},
number = {12},
pages = {125117},
abstract = {Quantum gas microscopes are a promising tool to study interacting quantum many-body systems and bridge the gap between theoretical models and real materials. So far, they were limited to measurements of instantaneous correlation functions of the form ⟨ˆO(t)⟩, even though extensions to frequency-resolved response functions ⟨ˆO(t)ˆO(0)⟩ would provide important information about the elementary excitations in a many-body system. For example, single-particle spectral functions, which are usually measured using photoemission experiments in electron systems, contain direct information about fractionalization and the quasiparticle excitation spectrum. Here, we propose a measurement scheme to experimentally access the momentum and energy-resolved spectral function in a quantum gas microscope with currently available techniques. As an example for possible applications, we numerically calculate the spectrum of a single hole excitation in one-dimensional t−J models with isotropic and anisotropic antiferromagnetic couplings. A sharp asymmetry in the distribution of spectral weight appears when a hole is created in an isotropic Heisenberg spin chain. This effect slowly vanishes for anisotropic spin interactions and disappears completely in the case of pure Ising interactions. The asymmetry strongly depends on the total magnetization of the spin chain, which can be tuned in experiments with quantum gas microscopes. An intuitive picture for the observed behavior is provided by a slave-fermion mean-field theory. The key properties of the spectra are visible at currently accessible temperatures.},
web_url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.125117},
DOI = {10.1103/PhysRevB.97.125117},
author = {Bohrdt, A. and Greif, D. and Demler, Eugene and Knap, Michael and Grusdt, F.}
}
@Article { Hütten_Nat.Comm.2018,
title = {Ultrafast quantum control of ionization dynamics in krypton},
journal = {Nature Communications},
year = {2018},
month = {2},
day = {19},
volume = {9},
number = {719},
abstract = {Ultrafast spectroscopy with attosecond resolution has enabled the real time observation of ultrafast electron dynamics in atoms, molecules and solids. These experiments employ attosecond pulses or pulse trains and explore dynamical processes in a pump–probe scheme that is selectively sensitive to electronic state of matter via photoelectron or XUV absorption spectroscopy or that includes changes of the ionic state detected via photo-ion mass spectrometry. Here, we demonstrate how the implementation of combined photo-ion and absorption spectroscopy with attosecond resolution enables tracking the complex multidimensional excitation and decay cascade of an Auger auto-ionization process of a few femtoseconds in highly excited krypton. In tandem with theory, our study reveals the role of intermediate electronic states in the formation of multiply charged ions. Amplitude tuning of a dressing laser field addresses different groups of decay channels and allows exerting temporal and quantitative control over the ionization dynamics in rare gas atoms.},
web_url = {https://www.nature.com/articles/s41467-018-03122-1},
DOI = {10.1038/s41467-018-03122-1},
author = {H{\"u}tten, Konrad and Mittermair, Michael and Stock, Sebastian O. and Beerwerth, Randolf and Shirvanyan, Vahe and Riemensberger, Johann and Duensing, Andreas and Heider, Rupert and Wagner, Martin S. and Guggenmos, Alexander and Fritzsche, Stephan and Kabachnik, Nikolay M. and Kienberger, Reinhard and Bernhardt, Brigitta}
}
@Article { Welte_Phys.Rev.X2018,
title = {Photon-Mediated Quantum Gate between Two Neutral Atoms in an Optical Cavity},
journal = {Physics Review X},
year = {2018},
month = {2},
day = {6},
volume = {8},
pages = {011018},
abstract = {Quantum logic gates are fundamental building blocks of quantum computers. Their integration into quantum networks requires strong qubit coupling to network channels, as can be realized with neutral atoms and optical photons in cavity quantum electrodynamics. Here we demonstrate that the long-range interaction mediated by a flying photon performs a gate between two stationary atoms inside an optical cavity from which the photon is reflected. This single step executes the gate in 2 \(\mu\)s. We show an entangling operation between the two atoms by generating a Bell state with 76(2)\% fidelity. The gate also operates as a cnot. We demonstrate 74.1(1.6)\% overlap between the observed and the ideal gate output, limited by the state preparation fidelity of 80.2(0.8)\%. As the atoms are efficiently connected to a photonic channel, our gate paves the way towards quantum networking with multiqubit nodes and the distribution of entanglement in repeater-based long-distance quantum networks.},
web_url = {https://journals.aps.org/prx/abstract/10.1103/PhysRevX.8.011018},
DOI = {10.1103/PhysRevX.8.011018},
author = {Welte, Stephan and Hacker, Bastian and Daiss, Severin and Ritter, Stephan and Rempe, Gerhard}
}
@Article { Seifert_NatComm2018,
title = {Spin Hall photoconductance in a three-dimensional topological insulator at room temperature},
journal = {Nature Communications},
year = {2018},
month = {1},
day = {23},
volume = {9},
number = {331},
abstract = {Three-dimensional topological insulators are a class of Dirac materials, wherein strong spin-orbit coupling leads to two-dimensional surface states. The latter feature spin-momentum locking, i.e., each momentum vector is associated with a spin locked perpendicularly to it in the surface plane. While the principal spin generation capability of topological insulators is well established, comparatively little is known about the interaction of the spins with external stimuli like polarized light. We observe a helical, bias-dependent photoconductance at the lateral edges of topological Bi2Te2Se platelets for perpendicular incidence of light. The same edges exhibit also a finite bias-dependent Kerr angle, indicative of spin accumulation induced by a transversal spin Hall effect in the bulk states of the Bi2Te2Se platelets. A symmetry analysis shows that the helical photoconductance is distinct to common longitudinal photoconductance and photocurrent phenomena, but consistent with optically injected spins being transported in the side facets of the platelets.},
web_url = {https://www.nature.com/articles/s41467-017-02671-1},
DOI = {10.1038/s41467-017-02671-1},
author = {Seifert, Paul and Vaklinova, Kristina and Ganichev, Sergey and Kern, Klaus and Burghard, Marko and Holleitner, Alexander W.}
}
@Report { Schmidt_Reports_on_Progress2018,
title = {Universal many-body response of heavy impurities coupled to a Fermi sea: a review of recent progress},
journal = {Reports on Progress in Physics},
year = {2018},
month = {1},
day = {5},
volume = {81},
number = {2},
pages = {38},
abstract = {In this report we discuss the dynamical response of heavy quantum impurities immersed in a Fermi gas at zero and at finite temperature. Studying both the frequency and the time domain allows one to identify interaction regimes that are characterized by distinct many-body dynamics. From this theoretical study a picture emerges in which impurity dynamics is universal on essentially all time scales, and where the high-frequency few-body response is related to the long-time dynamics of the Anderson orthogonality catastrophe by Tan relations. Our theoretical description relies on different and complementary approaches: functional determinants give an exact numerical solution for time- and frequency-resolved responses, bosonization provides accurate analytical expressions at low temperatures, and the theory of Toeplitz determinants allows one to analytically predict response up to high temperatures. Using these approaches we predict the thermal decoherence rate of the fermionic system and prove that within the considered model the fastest rate of long-time decoherence is given by \(\gamma\)=\(\pi\)kB T∕4. We show that Feshbach resonances in cold atomic systems give access to new interaction regimes where quantum effects can prevail even in the thermal regime of many-body dynamics. The key signature of this phenomenon is a crossover between different exponential decay rates of the real-time Ramsey signal. It is shown that the physics of the orthogonality catastrophe is experimentally observable up to temperatures T∕TF\(\le\) 0.2 where it leaves its fingerprint in a power-law temperature dependence of thermal spectral weight and we review how this phenomenon is related to the physics of heavy ions in liquid 3 He and the formation of Fermi polarons. The presented results are in excellent agreement with recent experiments on LiK mixtures, and we predict several new phenomena that can be tested using currently available experimental technology.},
web_url = {http://iopscience.iop.org/article/10.1088/1361-6633/aa9593/meta},
DOI = {10.1088/1361-6633/aa9593},
author = {Schmidt, Richard and Knap, Michael and Ivanov, Dmitri A and You, Jhih-Shih and Cetina, Marko and Demler, Eugene}
}
@Article { Lohse_Nature2018,
title = {Exploring 4D quantum Hall physics with a 2D topological charge pump},
journal = {Nature},
year = {2018},
month = {1},
day = {4},
volume = {553},
pages = {55-58},
abstract = {The discovery of topological states of matter has greatly improved our understanding of phase transitions in physical systems. Instead of being described by local order parameters, topological phases are described by global topological invariants and are therefore robust against perturbations. A prominent example is the two-dimensional (2D) integer quantum Hall effect1: it is characterized by the first Chern number, which manifests in the quantized Hall response that is induced by an external electric field2. Generalizing the quantum Hall effect to four-dimensional (4D) systems leads to the appearance of an additional quantized Hall response, but one that is nonlinear and described by a 4D topological invariant—the second Chern number3,4. Here we report the observation of a bulk response with intrinsic 4D topology and demonstrate its quantization by measuring the associated second Chern number. By implementing a 2D topological charge pump using ultracold bosonic atoms in an angled optical superlattice, we realize a dynamical version of the 4D integer quantum Hall effect5,6. Using a small cloud of atoms as a local probe, we fully characterize the nonlinear response of the system via in situ imaging and site-resolved band mapping. Our findings pave the way to experimentally probing higher-dimensional quantum Hall systems, in which additional strongly correlated topological phases, exotic collective excitations and boundary phenomena such as isolated Weyl fermions are predicted4.},
web_url = {https://www.nature.com/articles/nature25000},
DOI = {10.1038/nature25000},
author = {Lohse, Michael and Schweizer, Christian and Price, Hannah M. and Zilberberg, Oded and Bloch, Immanuel}
}
@Article { Bausch_PNAS2018,
title = {Size-driven quantum phase transitions},
journal = {Proceedings of the National Academy of Sciences},
year = {2018},
month = {1},
volume = {115},
number = {1},
pages = {19-23},
abstract = {Can the properties of the thermodynamic limit of a many-body quantum system be extrapolated by analyzing a sequence of finite-size cases? We present models for which such an approach gives completely misleading results: translationally invariant, local Hamiltonians on a square lattice with open boundary conditions and constant spectral gap, which have a classical product ground state for all system sizes smaller than a particular threshold size, but a ground state with topological degeneracy for all system sizes larger than this threshold. Starting from a minimal case with spins of dimension 6 and threshold lattice size 15\(\times\)15, we show that the latter grows faster than any computable function with increasing local spin dimension. The resulting effect may be viewed as a unique type of quantum phase transition that is driven by the size of the system rather than by an external field or coupling strength. We prove that the construction is thermally robust, showing that these effects are in principle accessible to experimental observation.},
web_url = {http://www.pnas.org/content/115/1/19},
DOI = {10.1073/pnas.1705042114},
author = {Bausch, Johannes and Cubitt, Toby and Lucia, Angelo and Perez-Garcia, David and Wolf, Michael}
}
@Article { Beaud_J.Math.Phys2018,
title = {Bounds on the entanglement entropy of droplet states in the XXZ spin chain},
journal = {Journal of Mathematical Physics},
year = {2018},
month = {1},
volume = {59},
pages = {012109},
abstract = {We consider a class of one-dimensional quantum spin systems on the finite lattice \(\Lambda\)\(\subset\)ℤ, related to the XXZ spin chain in its Ising phase. It includes in particular the so-called droplet Hamiltonian. The entanglement entropy of energetically low-lying states over a bipartition \(\Lambda\) = B \(\cup\) Bc is investigated and proven to satisfy a logarithmic bound in terms of min\{n, |B|, |Bc|\}, where n denotes the maximal number of down spins in the considered state. Upon addition of any (positive) random potential, the bound becomes uniformly constant on average, thereby establishing an area law. The proof is based on spectral methods: a deterministic bound on the local (many-body integrated) density of states is derived from an energetically motivated Combes–Thomas estimate.},
web_url = {https://aip.scitation.org/doi/abs/10.1063/1.5007035},
DOI = {10.1063/1.5007035},
author = {Beaud, V. and Warzel, Simone}
}
@Article { Körber_Nat.Photonics2018,
title = {Decoherence-protected memory for a single-photon qubit},
journal = {Nature Photonics},
year = {2017},
month = {12},
day = {11},
volume = {12},
pages = {18-21},
abstract = {Distributed quantum computation in a quantum network is based on the idea that qubits can be preserved and efficiently exchanged between long-lived, stationary network nodes via photonic links4. Although long qubit lifetimes have been observed and non-qubit excitations have been memorized the long-lived storage and efficient retrieval of a photonic qubit by means of a light–matter interface remains an outstanding challenge. Here, we report on a qubit memory based on a single atom coupled to a high-finesse optical resonator. By mapping the qubit between an interface basis with strong light–matter coupling and a memory basis with low decoherence, we achieve a coherence time exceeding 100 ms with a time-independent storage-and-retrieval efficiency of 22\%. The former constitutes an improvement by two orders of magnitude and thus implements an efficient photonic qubit memory with a coherence time that exceeds the lower bound needed for direct qubit teleportation in a global quantum internet.},
web_url = {https://www.nature.com/articles/s41566-017-0050-y},
DOI = {doi:10.1038/s41566-017-0050-y},
author = {K{\"o}rber, Matthias and Morin, O. and Langenfeld, S. and Neuzner, A. and Ritter, Stephan and Rempe, Gerhard}
}
@Article { Aizenman_ActaPhysicaPolonicaA2017,
title = {Edge Switching Transformations of Quantum Graphs},
journal = {Acta Physica Polonica A},
year = {2017},
month = {12},
volume = {132},
number = {6},
pages = {1699-1703},
abstract = {Discussed here are the effects of basics graph transformations on the spectra of associated quantum graphs. In particular it is shown that under an edge switch the spectrum of the transformed Schr{\"o}dinger operator is interlaced with that of the original one. By implication, under edge swap the spectra before and after the transformation, denoted by \{Eₙ\}\verb=^=\{\(\infty\)\}ₙ₌₁ and \{Ẽₙ\}\verb=^=\{\(\infty\)\}ₙ₌₁ correspondingly, are level-2 interlaced, so that Eₙ-₂ \(\le\) Ẽₙ \(\le\) Eₙ₊₂. The proofs are guided by considerations of the quantum graphs' discrete analogs.},
web_url = {http://psjd.icm.edu.pl/psjd/element/bwmeta1.element.bwnjournal-article-appv132n6p09kz?q=bwmeta1.element.bwnjournal-number-appola-2017-132-6;8\&qt=CHILDREN-STATELESS},
DOI = {10.12693/APhysPolA.132.1699},
author = {Aizenman, Michael and Schanz, H. and Smilansky, U. and Warzel, Simone}
}
@Article { Bordia_Phys.Rev.X.2017,
title = {Probing Slow Relaxation and Many-Body Localization in Two-Dimensional Quasiperiodic Systems},
journal = {Physical Review X},
year = {2017},
month = {11},
day = {28},
volume = {7},
pages = {041047},
abstract = {In a many-body localized (MBL) quantum system, the ergodic hypothesis breaks down, giving rise to a fundamentally new many-body phase. Whether and under which conditions MBL can occur in higher dimensions remains an outstanding challenge both for experiments and theory. Here, we experimentally explore the relaxation dynamics of an interacting gas of fermionic potassium atoms loaded in a two-dimensional optical lattice with different quasiperiodic potentials along the two directions. We observe a dramatic slowing down of the relaxation for intermediate disorder strengths. Furthermore, beyond a critical disorder strength, we see negligible relaxation on experimentally accessible time scales, indicating a possible transition into a two-dimensional MBL phase. Our experiments reveal a distinct interplay of interactions, disorder, and dimensionality and provide insights into regimes where controlled theoretical approaches are scarce.},
web_url = {https://journals.aps.org/prx/abstract/10.1103/PhysRevX.7.041047},
DOI = {10.1103/PhysRevX.7.041047},
author = {Bordia, Pranjal and L{\"u}schen, Henrik and Scherg, Sebastian and Gopalakrishnan, Sarang and Knap, Michael and Schneider, Ulrich and Bloch, Immanuel}
}
@Article { Weidinger_Phys.Rev.B.2017,
title = {Dynamical quantum phase transitions in systems with continuous symmetry breaking},
journal = {Physics Review B},
year = {2017},
month = {10},
day = {30},
volume = {96},
pages = {134313},
abstract = {Interacting many-body systems that are driven far away from equilibrium can exhibit phase transitions between dynamically emerging quantum phases, which manifest as singularities in the Loschmidt echo. Whether and under which conditions such dynamical transitions occur in higher-dimensional systems with spontaneously broken continuous symmetries is largely elusive thus far. Here, we study the dynamics of the Loschmidt echo in the three-dimensional O(N) model following a quantum quench from a symmetry-breaking initial state. The O(N) model exhibits a dynamical transition in the asymptotic steady state, separating two phases with a finite and vanishing order parameter, that is associated with the broken symmetry. We analytically calculate the rate function of the Loschmidt echo and find that it exhibits periodic kink singularities when this dynamical steady-state transition is crossed. The singularities arise exactly at the zero crossings of the oscillating order parameter. As a consequence, the appearance of the kink singularities in the transient dynamics is directly linked to a dynamical transition in the order parameter. Furthermore, we argue, that our results for dynamical quantum phase transitions in the O(N) model are general and apply to generic systems with continuous symmetry breaking.},
web_url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.134313},
DOI = {10.1103/PhysRevB.96.134313},
author = {Weidinger, Simon A. and Heyl, Markus and Silva, Alessandro and Knap, Michael}
}
@Article { Joas_NatComm2017,
title = {Quantum sensing of weak radio-frequency signals by pulsed Mollow absorption spectroscopy},
journal = {Nat. Commun.},
year = {2017},
month = {10},
day = {17},
volume = {8},
pages = {964},
abstract = {Quantum sensors—qubits sensitive to external fields—have become powerful detectors for various small acoustic and electromagnetic fields. A major key to their success have been dynamical decoupling protocols which enhance sensitivity to weak oscillating (AC) signals. Currently, those methods are limited to signal frequencies below a few MHz. Here we harness a quantum-optical effect, the Mollow triplet splitting of a strongly driven two-level system, to overcome this limitation. We microscopically understand this effect as a pulsed dynamical decoupling protocol and find that it enables sensitive detection of fields close to the driven transition. Employing a nitrogen-vacancy center, we detect GHz microwave fields with a signal strength (Rabi frequency) below the current detection limit, which is set by the center’s spectral linewidth 1∕T2*. Pushing detection sensitivity to the much lower 1/T2 limit, this scheme could enable various applications, most prominently coherent coupling to single phonons and microwave photons.},
DOI = {10.1038/s41467-017-01158-3},
author = {Joas, T. and Waeber, A. M. and Braunbeck, G. and Reinhard, Friedemann}
}
@Article { Bravyi_arXiv2017,
title = {Correcting coherent errors with surface codes},
year = {2017},
month = {10},
day = {6},
abstract = {We study how well topological quantum codes can tolerate coherent noise caused by systematic unitary errors such as unwanted Z-rotations. Our main result is an efficient algorithm for simulating quantum error correction protocols based on the 2D surface code in the presence of coherent errors. The algorithm has runtime O(n2), where n is the number of physical qubits. It allows us to simulate systems with more than one thousand qubits and obtain the first error threshold estimates for several toy models of coherent noise. Numerical results are reported for storage of logical states subject to Z-rotation errors and for logical state preparation with general SU(2) errors. We observe that for large code distances the effective logical-level noise is well-approximated by random Pauli errors even though the physical-level noise is coherent. Our algorithm works by mapping the surface code to a system of Majorana fermions.},
web_url = {https://arxiv.org/abs/1710.02270},
state = {submitted},
author = {Bravyi, Sergey and Englbrecht, Matthias and K{\"o}nig, Robert and Peard, Nolan}
}
@Article { Gross_Science2017,
title = {Quantum simulations with ultracold atoms in optical lattices},
journal = {Science},
year = {2017},
month = {9},
day = {8},
volume = {357},
number = {6355},
pages = {995-1001},
abstract = {Abstract
Quantum simulation, a subdiscipline of quantum computation, can provide valuable insight into difficult quantum problems in physics or chemistry. Ultracold atoms in optical lattices represent an ideal platform for simulations of quantum many-body problems. Within this setting, quantum gas microscopes enable single atom observation and manipulation in large samples. Ultracold atom–based quantum simulators have already been used to probe quantum magnetism, to realize and detect topological quantum matter, and to study quantum systems with controlled long-range interactions. Experiments on many-body systems out of equilibrium have also provided results in regimes unavailable to the most advanced supercomputers. We review recent experimental progress in this field and comment on future directions.},
web_url = {http://science.sciencemag.org/content/357/6355/995.full},
DOI = {10.1126/science.aal3837},
author = {Gross, Christian and Bloch, Immanuel}
}
@Article { vonSoosten_math-ph2017,
title = {Non-Ergodic Delocalization in the Rosenzweig-Porter Model},
journal = {Mathematical Physics},
year = {2017},
month = {9},
abstract = {We consider the Rosenzweig-Porter model H=V+T−−\(\sqrt{}\)\(\Phi\), where V is a N\(\times\)N diagonal matrix, \(\Phi\) is drawn from the N\(\times\)N Gaussian Orthogonal Ensemble, and N−1≪T≪1. We prove that the eigenfunctions of H are typically supported in a set of approximately NT sites, thereby confirming the existence of a previously conjectured non-ergodic delocalized phase. Our proof is based on martingale estimates along the characteristic curves of the stochastic advection equation satisfied by the local resolvent of the Brownian motion representation of H.},
web_url = {https://arxiv.org/abs/1709.10313},
state = {submitted},
author = {von Soosten, P. and Warzel, Simone}
}
@Article { Knap_PRL2017,
title = {Noise-induced subdiffusion in strongly localized quantum systems},
journal = {Phys. Rev. Lett.},
year = {2017},
month = {7},
day = {26},
volume = {119},
pages = {046601},
abstract = {We consider the dynamics of strongly localized systems subject to dephasing noise with arbitrary correlation time. Although noise inevitably induces delocalization, transport in the noise-induced delocalized phase is subdiffusive in a parametrically large intermediate-time window. We argue for this intermediate-time subdiffusive regime both analytically and using numerical simulations on single-particle localized systems. Furthermore, we show that normal diffusion is restored in the long-time limit, through processes analogous to variable-range hopping. With numerical simulations based on Lanczos exact diagonalization, we demonstrate that our qualitative conclusions are also valid for interacting systems in the many-body localized phase.},
web_url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.046601},
DOI = {10.1103/PhysRevLett.119.046601},
author = {Gopalakrishnan, Sarang and Islam, K. Ranjibul and Knap, Michael}
}
@Article { quantumsensing2017,
title = {Quantum sensing},
journal = {Rev. Mod. Phys.},
year = {2017},
month = {7},
day = {25},
volume = {89},
number = {3},
abstract = {“Quantum sensing” describes the use of a quantum system, quantum properties, or quantum phenomena to perform a measurement of a physical quantity. Historical examples of quantum sensors include magnetometers based on superconducting quantum interference devices and atomic vapors or atomic clocks. More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions, and flux qubits. The field is expected to provide new opportunities—especially with regard to high sensitivity and precision—in applied physics and other areas of science. This review provides an introduction to the basic principles, methods, and concepts of quantum sensing from the viewpoint of the interested experimentalist.},
DOI = {10.1103/RevModPhys.89.035002},
author = {Degen, C. L. and Reinhard, Friedemann and Cappellaro, P}
}
@Article { Knap_PRB_2017,
title = {Theory of parametrically amplified electron-phonon superconductivity},
journal = {Phys. Rev. B},
year = {2017},
month = {7},
day = {19},
volume = {96},
pages = {014512},
abstract = {Ultrafast optical manipulation of ordered phases in strongly correlated materials is a topic of significant theoretical, experimental, and technological interest. Inspired by a recent experiment on light-induced superconductivity in fullerenes [M. Mitrano et al., Nature (London) 530, 461 (2016)], we develop a comprehensive theory of light-induced superconductivity in driven electron-phonon systems with lattice nonlinearities. In analogy with the operation of parametric amplifiers, we show how the interplay between the external drive and lattice nonlinearities lead to significantly enhanced effective electron-phonon couplings. We provide a detailed and unbiased study of the nonequilibrium dynamics of the driven system using the real-time Green's function technique. To this end, we develop a Floquet generalization of the Migdal-Eliashberg theory and derive a numerically tractable set of quantum Floquet-Boltzmann kinetic equations for the coupled electron-phonon system. We study the role of parametric phonon generation and electronic heating in destroying the transient superconducting state. Finally, we predict the transient formation of electronic Floquet bands in time- and angle-resolved photoemission spectroscopy experiments as a consequence of the proposed mechanism.},
note = {Editors' Suggestion},
web_url = {http://arxiv.org/abs/1702.02531},
DOI = {doi.org/10.1103/PhysRevB.96.014512},
author = {Babadi, Mehrtash and Knap, Michael and Martin, Ivar and Refael, Gil and Demler, Eugene}
}
@Article { Beaud_Ann.HenriPolonica2017,
title = {Low-Energy Fock-Space Localization for Attractive Hard-Core Particles in Disorder},
journal = {Annales Henri Poincar{\'e}},
year = {2017},
month = {6},
day = {20},
volume = {18},
number = {10},
pages = {3143–3166},
abstract = {We study a one-dimensional quantum system with an arbitrary number of hard-core particles on the lattice, which are subject to a deterministic attractive interaction as well as a random potential. Our choice of interaction is suggested by the spectral analysis of the XXZ quantum spin chain. The main result concerns a version of high-disorder Fock-space localization expressed here in the configuration space of hard-core particles. The proof relies on an energetically motivated Combes–Thomas estimate and an effective one-particle analysis. As an application, we show the exponential decay of the two-point function in the infinite system uniformly in the particle number.},
web_url = {https://link.springer.com/article/10.1007/s00023-017-0591-0},
DOI = {10.1007/s00023-017-0591-0},
author = {Beaud, V. and Warzel, Simone}
}
@Article { brandtdonor2017,
title = {Multiple-Quantum Transitions and Charge-Induced Decoherence of Donor Nuclear Spins in Silicon},
journal = {PRL},
year = {2017},
month = {6},
day = {15},
volume = {118},
abstract = {We study single- and multiquantum transitions of the nuclear spins of an ensemble of ionized arsenic donors in silicon and find quadrupolar effects on the coherence times, which we link to fluctuating electrical field gradients present after the application of light and bias voltage pulses. To determine the coherence times of superpositions of all orders in the 4-dimensional Hilbert space, we use a phase-cycling technique and find that, when electrical effects were allowed to decay, these times scale as expected for a fieldlike decoherence mechanism such as the interaction with surrounding 29Si nuclear spins.},
web_url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.246401},
DOI = {10.1103/PhysRevLett.118.246401},
author = {Franke, David P. and Pfl{\"u}ger, Moritz P. D. and Itoh, Kohei M. and Brandt, Martin S.}
}
@Unknown { Bohrst_NJP_2017,
title = {Scrambling and thermalization in a diffusive quantum many-body system},
journal = {New J. Phys.},
year = {2017},
month = {6},
day = {2},
volume = {19},
pages = {063001},
abstract = {Out-of-time ordered (OTO) correlation functions describe scrambling of information in correlated quantum matter. They are of particular interest in incoherent quantum systems lacking well defined quasi-particles. Thus far, it is largely elusive how OTO correlators spread in incoherent systems with diffusive transport governed by a few globally conserved quantities. Here, we study the dynamical response of such a system using high-performance matrix-product-operator techniques. Specifically, we consider the non-integrable, one-dimensional Bose–Hubbard model in the incoherent high-temperature regime. Our system exhibits diffusive dynamics in time-ordered correlators of globally conserved quantities, whereas OTO correlators display a ballistic, light-cone spreading of quantum information. The slowest process in the global thermalization of the system is thus diffusive, yet information spreading is not inhibited by such slow dynamics. We furthermore develop an experimentally feasible protocol to overcome some challenges faced by existing proposals and to probe time-ordered and OTO correlation functions. Our study opens new avenues for both the theoretical and experimental exploration of thermalization and information scrambling dynamics.},
web_url = {http://arxiv.org/abs/1612.02434},
DOI = {dx.doi.org/10.1088/1367-2630/aa719b},
author = {Bohrdt, A. and Mendl, C. B. and Endres, Manuel and Knap, Michael}
}
@Article { Meinert_Science_2017,
title = {Bloch oscillations in the absence of a lattice.},
journal = {Science},
year = {2017},
month = {6},
day = {2},
volume = {356},
pages = {945},
abstract = {The interplay of strong quantum correlations and far-from-equilibrium conditions can give rise to striking dynamical phenomena. We experimentally investigated the quantum motion of an impurity atom immersed in a strongly interacting one-dimensional Bose liquid and subject to an external force. We found that the momentum distribution of the impurity exhibits characteristic Bragg reflections at the edge of an emergent Brillouin zone. Although Bragg reflections are typically associated with lattice structures, in our strongly correlated quantum liquid they result from the interplay of short-range crystalline order and kinematic constraints on the many-body scattering processes in the one-dimensional system. As a consequence, the impurity exhibits periodic dynamics, reminiscent of Bloch oscillations, although the quantum liquid is translationally invariant. Our observations are supported by large-scale numerical simulations.},
DOI = {10.1126/science.aah6616},
author = {Meinert, Florian and Knap, Michael and Kirilov, Emil and Jag-Lauber, Katharina and Zvonarev, Mikhail B. and Demler, Eugene and N{\"a}gerl, Hanns-Christoph}
}
@Article { vonSoosten_math-ph20170,
title = {The Localization Transition in the Ultrametric Ensemble},
journal = {Mathematical Physics},
year = {2017},
month = {5},
day = {11},
abstract = {We study the hierarchical analogue of power-law random band matrices, a symmetric ensemble of random matrices with independent entries whose variances decay exponentially in the metric induced by the tree topology on N. We map out the entirety of the localization regime by proving the localization of eigenfunctions and Poisson statistics of the suitably scaled eigenvalues. Our results complement existing works on complete delocalization and random matrix universality, thereby proving the existence of a phase transition in this model.},
web_url = {https://arxiv.org/abs/1705.04165},
state = {submitted},
author = {von Soosten, P. and Warzel, Simone}
}
@Article { knapfloquet20170,
title = {Floquet prethermalization and regimes of heating in a periodically driven, interacting quantum system.},
journal = {Sci. Rep.},
year = {2017},
month = {4},
day = {3},
volume = {7},
pages = {45382},
abstract = {We study the regimes of heating in the periodically driven O(N)-model, which represents a generic model for interacting quantum many-body systems. By computing the absorbed energy with a non-equilibrium Keldysh Green's function approach, we establish three dynamical regimes: at short times a single-particle dominated regime, at intermediate times a stable Floquet prethermal regime in which the system ceases to absorb, and at parametrically late times a thermalizing regime. Our simulations suggest that in the thermalizing regime the absorbed energy grows algebraically in time with an the exponent that approaches the universal value of 1/2, and is thus significantly slower than linear Joule heating. Our results demonstrate the parametric stability of prethermal states in a generic many-body system driven at frequencies that are comparable to its microscopic scales. This paves the way for realizing exotic quantum phases, such as time crystals or interacting topological phases, in the prethermal regime of interacting Floquet systems.},
DOI = {10.1038/srep45382},
author = {Weidinger, Simon A. and Knap, Michael}
}
@Article { Bravyi_arXiv20170,
title = {Quantum advantage with shallow circuits},
year = {2017},
month = {4},
day = {3},
abstract = {We prove that constant-depth quantum circuits are more powerful than their classical counterparts. To this end we introduce a non-oracular version of the Bernstein-Vazirani problem which we call the 2D Hidden Linear Function problem. An instance of the problem is specified by a quadratic form q that maps n-bit strings to integers modulo four. The goal is to identify a linear boolean function which describes the action of q on a certain subset of n-bit strings. We prove that any classical probabilistic circuit composed of bounded fan-in gates that solves the 2D Hidden Linear Function problem with high probability must have depth logarithmic in n. In contrast, we show that this problem can be solved with certainty by a constant-depth quantum circuit composed of one- and two-qubit gates acting locally on a two-dimensional grid.},
web_url = {Quantum advantage with shallow circuits},
state = {submitted},
author = {Bravyi, Sergey and Gosset, David and K{\"o}nig, Robert}
}
@Article { Knapmbl2017,
title = {Rare region effects and dynamics near the many-body localization transition.},
journal = {Annalen der Physik, Special issue on Many-Body Localization},
year = {2017},
month = {1},
day = {12},
abstract = {The low-frequency response of systems near the many-body localization phase transition, on either side of the transition, is dominated by contributions from rare regions that are locally “in the other phase”, i.e., rare localized regions in a system that is typically thermal, or rare thermal regions in a system that is typically localized. Rare localized regions affect the properties of the thermal phase, especially in one dimension, by acting as bottlenecks for transport and the growth of entanglement, whereas rare thermal regions in the localized phase act as local “baths” and dominate the low-frequency response of the MBL phase. We review recent progress in understanding these rare-region effects, and discuss some of the open questions associated with them: in particular, whether and in what circumstances a single rare thermal region can destabilize the many-body localized phase.},
DOI = {10.1002/andp.201600326},
author = {Agarwal, Kartiek and Altman, Ehud and Demler, Eugene and Gopalakrishnan, Sarang and Huse, David A. and Knap, Michael}
}
@Article { demler2016,
title = {Dynamical Cooper pairing in non-equilibrium electron-phonon systems.},
journal = {Phys. Rev. B},
year = {2016},
month = {12},
day = {2},
volume = {94},
abstract = {We analyze Cooper pairing instabilities in strongly driven electron-phonon systems. The light-induced nonequilibrium state of phonons results in a simultaneous increase of the superconducting coupling constant and the electron scattering. We demonstrate that the competition between these effects leads to an enhanced superconducting transition temperature in a broad range of parameters. Our results may explain the observed transient enhancement of superconductivity in several classes of materials upon irradiation with high intensity pulses of terahertz light, and may pave new ways for engineering high-temperature light-induced superconducting states.},
DOI = {10.1103/PhysRevB.94.214504},
author = {Knap, Michael and Babadi, Mehrtash and Refael, Gil and Martin, Ivar and Demler, Eugene}
}
@Article { Punk2016,
title = {Finite-temperature scaling close to Ising-nematic quantum critical points in two-dimensional metals},
journal = {Phys. Rev. B},
year = {2016},
month = {11},
day = {7},
volume = {94},
number = {195113},
abstract = {We study finite-temperature properties of metals close to an Ising-nematic quantum critical point in two spatial dimensions. In particular we show that at any finite temperature there is a regime where order parameter fluctuations are characterized by a dynamical critical exponent z=2, in contrast to z=3 found at zero temperature. Our results are based on a simple Eliashberg-type approach, which gives rise to a boson self-energy proportional to \(\Omega\)/\(\gamma\)(T) at small momenta, where \(\gamma\)(T) is the temperature dependent fermion scattering rate. These findings might shed some light on recent Monte Carlo simulations at finite temperature, where results consistent with z=2 were found.},
DOI = {10.1103/PhysRevB.94.195113},
author = {Punk, Matthias}
}
@Article { CetinaJLFWGLPSKD2016,
title = {Ultrafast many-body interferometry of impurities coupled to a Fermi sea},
journal = {Science},
year = {2016},
month = {10},
day = {7},
volume = {354},
number = {6308},
pages = {96-99},
abstract = {The fastest possible collective response of a quantum many-body system is related to its excitations at the highest possible energy. In condensed matter systems, the time scale for such “ultrafast” processes is typically set by the Fermi energy. Taking advantage of fast and precise control of interactions between ultracold atoms, we observed nonequilibrium dynamics of impurities coupled to an atomic Fermi sea. Our interferometric measurements track the nonperturbative quantum evolution of a fermionic many-body system, revealing in real time the formation dynamics of quasi-particles and the quantum interference between attractive and repulsive states throughout the full depth of the Fermi sea. Ultrafast time-domain methods applied to strongly interacting quantum gases enable the study of the dynamics of quantum matter under extreme nonequilibrium conditions.},
DOI = {10.1126/science.aaf5134},
author = {Cetina, Marko and Jag, Michael and Lous, Rianne S. and Fritsche, Isabella and Walraven, Jook T. M. and Grimm, Rudolf and Levinsen, Jesper and Parish, Meera M. and Schmidt, Richard and Knap, Michael and Demler, Eugene}
}
@Article { WildGKYL2016,
title = {Adiabatic Quantum Search in Open Systems},
journal = {Phys. Rev. Lett.},
year = {2016},
month = {10},
day = {6},
volume = {117},
number = {150501},
abstract = {Adiabatic quantum algorithms represent a promising approach to universal quantum computation. In isolated systems, a key limitation to such algorithms is the presence of avoided level crossings, where gaps become extremely small. In open quantum systems, the fundamental robustness of adiabatic algorithms remains unresolved. Here, we study the dynamics near an avoided level crossing associated with the adiabatic quantum search algorithm, when the system is coupled to a generic environment. At zero temperature, we find that the algorithm remains scalable provided the noise spectral density of the environment decays sufficiently fast at low frequencies. By contrast, higher order scattering processes render the algorithm inefficient at any finite temperature regardless of the spectral density, implying that no quantum speedup can be achieved. Extensions and implications for other adiabatic quantum algorithms will be discussed.},
DOI = {10.1103/PhysRevLett.117.150501},
author = {Wild, Dominik S. and Gopalakrishnan, Sarang and Knap, Michael and Yao, Norman Y. and Lukin, Mikhail D.}
}
@Article { König_Phys.Rev.Lett2016,
title = {Matrix Product Approximations to Multipoint Functions in Two-Dimensional Conformal Field Theory},
journal = {Physical Review Letters},
year = {2016},
month = {9},
day = {14},
volume = {117},
pages = {121601},
abstract = {Matrix product states (MPSs) illustrate the suitability of tensor networks for the description of interacting many-body systems: ground states of gapped 1D systems are approximable by MPSs, as shown by Hastings [M. B. Hastings, J. Stat. Mech. (2007) P08024]. By contrast, whether MPSs and more general tensor networks can accurately reproduce correlations in critical quantum systems or quantum field theories has not been established rigorously. Ample evidence exists: entropic considerations provide restrictions on the form of suitable ansatz states, and numerical studies show that certain tensor networks can indeed approximate the associated correlation functions. Here, we provide a complete positive answer to this question in the case of MPSs and 2D conformal field theory: we give quantitative estimates for the approximation error when approximating correlation functions by MPSs. Our work is constructive and yields an explicit MPS, thus providing both suitable initial values and a rigorous justification of variational methods.},
web_url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.121601},
DOI = {10.1103/PhysRevLett.117.121601},
author = {K{\"o}nig, Robert and Scholz, Volkher B.}
}
@Article { GopalakrishnanKD2016,
title = {Regimes of heating and dynamical response in driven many-body localized systems},
journal = {Phys. Rev. B},
year = {2016},
month = {9},
day = {14},
volume = {94},
number = {094201},
abstract = {We explore the response of many-body localized (MBL) systems to periodic driving of arbitrary amplitude, focusing on the rate at which they exchange energy with the drive. To this end, we introduce an infinite-temperature generalization of the effective “heating rate” in terms of the spread of a random walk in energy space. We compute this heating rate numerically and estimate it analytically in various regimes. When the drive amplitude is much smaller than the frequency, this effective heating rate is given by linear response theory with a coefficient that is proportional to the optical conductivity; in the opposite limit, the response is nonlinear and the heating rate is a nontrivial power law of time. We discuss the mechanisms underlying this crossover in the MBL phase. We comment on implications for the subdiffusive thermal phase near the MBL transition, and for response in imperfectly isolated MBL systems.},
DOI = {10.1103/PhysRevB.94.094201},
author = {Gopalakrishnan, Sarang and Knap, Michael and Demler, Eugene}
}
@Article { HalimehP2016,
title = {Spin structure factors of chiral quantum spin liquids on the kagome lattice},
journal = {Phys. Rev. B},
year = {2016},
month = {9},
day = {13},
volume = {94},
number = {104413},
abstract = {We calculate dynamical spin structure factors for gapped chiral spin liquid states in the spin-1/2 Heisenberg antiferromagnet on the kagome lattice using Schwinger-boson mean-field theory. In contrast to static (equal-time) structure factors, the dynamical structure factor shows clear signatures of time-reversal symmetry breaking for chiral spin liquid states. In particular, momentum inversion k→−k symmetry as well as the sixfold rotation symmetry around the \(\Gamma\) point are lost. We highlight other interesting features, such as a relatively flat onset of the two-spinon continuum for the cuboc1 state. Our work is based on the projective symmetry group classification of time-reversal symmetry breaking Schwinger-boson mean-field states by Messio, Lhuillier, and Misguich.},
DOI = {10.1103/PhysRevB.94.104413},
author = {Halimeh, Jad C. and Punk, Matthias}
}
@Article { FournaisLLO2016,
title = {Coulomb potentials and Taylor expansions in time-dependent density-functional theory},
journal = {Phys. Rev. A},
year = {2016},
month = {6},
day = {23},
volume = {92},
number = {062510},
abstract = {We investigate when Taylor expansions can be used to prove the Runge-Gross theorem, which is at the foundation of time-dependent density-functional theory (TDDFT). We start with a general analysis of the conditions for the Runge-Gross argument, especially the time differentiability of the density. The latter should be questioned in the presence of singular (e.g., Coulomb) potentials. Then we show that a singular potential in a one-body operator considerably decreases the class of time-dependent external potentials to which the original argument can be applied. A two-body singularity has an even stronger impact and an external potential is essentially incompatible with it. For the Coulomb interaction and all reasonable initial many-body states, the Taylor expansion only exists to a finite order, except for constant external potentials. Therefore, high-order Taylor expansions are not the right tool to study atoms and molecules in TDDFT.},
DOI = {10.1103/PhysRevA.93.062510},
author = {Fournais, S{\o}ren and Lampert, Jonas and Lewin, Mathieu and {\O}stergaard S{\o}rensen, Thomas}
}
@Article { HofmannNKCH2016,
title = {Ubiquity of Exciton Localization in Cryogenic Carbon Nanotubes},
journal = {Nano Lett.},
year = {2016},
month = {4},
day = {22},
volume = {16},
number = {5},
pages = {2958–2962},
abstract = {We present photoluminescence studies of individual semiconducting single-wall carbon nanotubes at room and cryogenic temperatures. From the analysis of spatial and spectral features of nanotube photoluminescence, we identify characteristic signatures of unintentional exciton localization. Moreover, we quantify the energy scale of exciton localization potentials as ranging from a few to a few tens of millielectronvolts and stemming from both environmental disorder and shallow covalent side-wall defects. Our results establish disorder-induced crossover from the diffusive to the localized regime of nanotube excitons at cryogenic temperatures as a ubiquitous phenomenon in micelle-encapsulated and as-grown carbon nanotubes.},
DOI = {10.1021/acs.nanolett.5b04901},
author = {Hofmann, Matthias and Noe, Jonathan and Kneer, Alexander and Crochet, Jared and H{\"o}gele, Alexander}
}
@Article { GopalakrishnanADHK2016,
title = {Griffiths effects and slow dynamics in nearly many-body localized systems},
journal = {Phys. Rev. B},
year = {2016},
month = {4},
day = {11},
volume = {93},
number = {134206},
abstract = {The low-frequency response of systems near a many-body localization transition can be dominated by rare regions that are locally critical or “in the other phase.” It is known that in one dimension, these rare regions can cause the dc conductivity and diffusion constant to vanish even inside the delocalized thermal phase. Here, we present a general analysis of such Griffiths effects in the thermal phase near the many-body localization transition: we consider both one-dimensional and higher-dimensional systems, subject to quenched randomness, and discuss both linear response (including the frequency- and wave-vector-dependent conductivity) and more general dynamics. In all the regimes we consider, we identify observables that are dominated by rare-region effects. In some cases (one-dimensional systems and Floquet systems with no extensive conserved quantities), essentially all long-time local observables are dominated by rare-region effects; in others, generic observables are instead dominated by hydrodynamic long-time tails throughout the thermal phase, and one must look at specific probes, such as spin echo, to see Griffiths behavior.},
author = {Gopalakrishnan, Sarang and Agarwal, Kartiek and Demler, Eugene A. and Huse, David A. and Knap, Michael}
}
@Unknown { Beverland_J.Maths.Phys._2016,
title = {Protected gates for topological quantum field theories},
journal = {Journal of Mathematical Physics},
year = {2016},
month = {1},
day = {13},
volume = {57},
pages = {022201},
abstract = {We study restrictions on locality-preserving unitary logical gates for topological quantum codes in two spatial dimensions. A locality-preserving operation is one which maps local operators to local operators — for example, a constant-depth quantum circuit of geometrically local gates, or evolution for a constant time governed by a geometrically local bounded-strength Hamiltonian. Locality-preserving logical gates of topological codes are intrinsically fault tolerant because spatially localized errors remain localized, and hence sufficiently dilute errors remain correctable. By invoking general properties of two-dimensional topological field theories, we find that the locality-preserving logical gates are severely limited for codes which admit non-abelian anyons, in particular, there are no locality-preserving logical gates on the torus or the sphere with M punctures if the braiding of anyons is computationally universal. Furthermore, for Ising anyons on the M-punctured sphere, locality-preserving gates must be elements of the logical Pauli group. We derive these results by relating logical gates of a topological code to automorphisms of the Verlinde algebra of the corresponding anyon model, and by requiring the logical gates to be compatible with basis changes in the logical Hilbert space arising from local F-moves and the mapping class group.},
web_url = {https://aip.scitation.org/doi/10.1063/1.4939783},
DOI = {10.1063/1.4939783},
author = {Beverland, Michael E. and Buerschaper, Oliver and K{\"o}nig, Robert}
}
@Article { TsvelikY2015,
title = {Quantum Phase Transition and Protected Ideal Transport in a Kondo Chain},
journal = {PRL},
year = {2015},
month = {11},
day = {20},
volume = {115},
number = {216402},
abstract = {We study the low energy physics of a Kondo chain where electrons from a one-dimensional band interact
with magnetic moments via an anisotropic exchange interaction. It is demonstrated that the anisotropy
gives rise to two different phases which are separated by a quantum phase transition. In the phase with easy
plane anisotropy, Z2 symmetry between sectors with different helicity of the electrons is broken. As a
result, localization effects are suppressed and the dc transport acquires (partial) symmetry protection.
This effect is similar to the protection of the edge transport in time-reversal invariant topological insulators.
The phase with easy axis anisotropy corresponds to the Tomonaga-Luttinger liquid with a pronounced
spin-charge separation. The slow charge density wave modes have no protection against localization.},
DOI = {10.1103/PhysRevLett.115.216402},
author = {Tsvelik, A. M. and Yevtushenko, O. M.}
}
@Article { LiuCDEPP2015,
title = {Massive Goldstone (Higgs) mode in two-dimensional ultracold atomic lattice systems},
journal = {Phys. Rev. B},
year = {2015},
month = {11},
day = {15},
volume = {92},
number = {174521},
abstract = {We discuss how to reveal the massive Goldstone mode, often referred to as the Higgs amplitude mode, near the superfluid-to-insulator quantum critical point (QCP) in a system of two-dimensional ultracold bosonic atoms in optical lattices. The spectral function of the amplitude response is obtained by analytic continuation of the kinetic energy correlation function calculated by Monte Carlo methods. Our results enable a direct comparison with the recent experiment [M. Endres, T. Fukuhara, D. Pekker, M. Cheneau, P. Schau{\ss}, C. Gross, E. Demler, S. Kuhr, and I. Bloch, Nature (London) 487, 454 (2012)] and demonstrate a good agreement for temperature shifts induced by lattice modulation. Based on our numerical analysis, we formulate the necessary conditions in terms of homogeneity, detuning from the QCP and temperature in order to reveal the massive Goldstone resonance peak in spectral functions experimentally. We also propose to apply a local modulation at the trap center to overcome the inhomogeneous broadening caused by the parabolic trap confinement.},
DOI = {10.1103/PhysRevB.92.174521},
author = {Liu, Longxiang and Chen, Kun and Deng, Youjin and Endress, Manuel and Pollet, Lode and Prokof'ev, Nikolay}
}
@Article { BukovGKD2015,
title = {Prethermal Floquet Steady States and Instabilities in the Periodically Driven, Weakly Interacting Bose-Hubbard Model},
journal = {Phys. Rev. Lett.},
year = {2015},
month = {11},
day = {11},
volume = {115},
number = {205301},
abstract = {We explore prethermal Floquet steady states and instabilities of the weakly interacting two-dimensional Bose-Hubbard model subject to periodic driving. We develop a description of the nonequilibrium dynamics, at arbitrary drive strength and frequency, using a weak-coupling conserving approximation. We establish the regimes in which conventional (zero-momentum) and unconventional [(\(\pi\),\(\pi\))-momentum] condensates are stable on intermediate time scales. We find that condensate stability is enhanced by increasing the drive strength, because this decreases the bandwidth of quasiparticle excitations and thus impedes resonant absorption and heating. Our results are directly relevant to a number of current experiments with ultracold bosons.},
DOI = {10.1103/PhysRevLett.115.205301},
author = {Bukov, Marin and Gopalakrishnan, Sarang and Knap, Michael and Demler, Eugene}
}
@Article { BabadiDK2015,
title = {Far-from-Equilibrium Field Theory of Many-Body Quantum Spin Systems: Prethermalization and Relaxation of Spin Spiral States in Three Dimensions},
journal = {Phys. Rev. X},
year = {2015},
month = {10},
day = {12},
volume = {5},
number = {041005},
abstract = {We study theoretically the far-from-equilibrium relaxation dynamics of spin spiral states in the three-dimensional isotropic Heisenberg model. The investigated problem serves as an archetype for understanding quantum dynamics of isolated many-body systems in the vicinity of a spontaneously broken continuous symmetry. We present a field-theoretical formalism that systematically improves on the mean field for describing the real-time quantum dynamics of generic spin-1/2 systems. This is achieved by mapping spins to Majorana fermions followed by a 1/N expansion of the resulting two-particle-irreducible effective action. Our analysis reveals rich fluctuation-induced relaxation dynamics in the unitary evolution of spin spiral states. In particular, we find the sudden appearance of long-lived prethermalized plateaus with diverging lifetimes as the spiral winding is tuned toward the thermodynamically stable ferro- or antiferromagnetic phases. The emerging prethermalized states are characterized by different bosonic modes being thermally populated at different effective temperatures and by a hierarchical relaxation process reminiscent of glassy systems. Spin-spin correlators found by solving the nonequilibrium Bethe-Salpeter equation provide further insight into the dynamic formation of correlations, the fate of unstable collective modes, and the emergence of fluctuation-dissipation relations. Our predictions can be verified experimentally using recent realizations of spin spiral states with ultracold atoms in a quantum gas microscope [S. Hild et al., Phys. Rev. Lett. 113, 147205 (2014)].},
DOI = {10.1103/PhysRevX.5.041005},
author = {Babadi, Mehrtash and Demler, Eugene and Knap, Michael}
}
@Article { SchaussZFHCMPBG2015,
title = {Microscopic Characterization of Scalable Coherent Rydberg Superatoms},
journal = {Physical Review X},
year = {2015},
month = {8},
day = {12},
volume = {5},
number = {031015},
abstract = {Strong interactions can amplify quantum effects such that they become important on macroscopic scales. Controlling these coherently on a single-particle level is essential for the tailored preparation of strongly correlated quantum systems and opens up new prospects for quantum technologies. Rydberg atoms offer such strong interactions, which lead to extreme nonlinearities in laser-coupled atomic ensembles. As a result, multiple excitation of a micrometer-sized cloud can be blocked while the light-matter coupling becomes collectively enhanced. The resulting two-level system, often called a “superatom,” is a valuable resource for quantum information, providing a collective qubit. Here, we report on the preparation of 2 orders of magnitude scalable superatoms utilizing the large interaction strength provided by Rydberg atoms combined with precise control of an ensemble of ultracold atoms in an optical lattice. The latter is achieved with sub-shot-noise precision by local manipulation of a two-dimensional Mott insulator. We microscopically confirm the superatom picture by in situ detection of the Rydberg excitations and observe the characteristic square-root scaling of the optical coupling with the number of atoms. Enabled by the full control over the atomic sample, including the motional degrees of freedom, we infer the overlap of the produced many-body state with a W state from the observed Rabi oscillations and deduce the presence of entanglement. Finally, we investigate the breakdown of the superatom picture when two Rydberg excitations are present in the system, which leads to dephasing and a loss of coherence.},
DOI = {10.1103/PhysRevX.5.031015},
author = {Zeiher, J. and Schauss, P. and Hild, S. and Macri, T. and Bloch, I. and Gross, C.}
}
@Article { FukuharaHZSBEG2015_2,
title = {Spatially Resolved Detection of a Spin-Entanglement Wave in a Bose-Hubbard Chain},
journal = {Physical Review Letters},
year = {2015},
month = {7},
day = {13},
volume = {115},
number = {035302},
abstract = {Entanglement is an essential property of quantum many-body systems. However, its local detection is challenging and was so far limited to spin degrees of freedom in ion chains. Here we measure entanglement between the spins of atoms located on two lattice sites in a one-dimensional BoseHubbard chain which features both local spin- and particle-number ﬂuctuations. Starting with an initially localized spin impurity, we observe an outwards propagating entanglement wave and show quantitatively how entanglement in the spin sector rapidly decreases with increasing particle-number ﬂuctuations in the chain.},
DOI = {10.1103/PhysRevLett.115.035302},
author = {Fukuhara, T. and Hild, S. and Zeiher, J. and Schauss, P. and Bloch, I. and Endres, M. and Gross, C.}
}
@Article { BruognolovW2015,
title = {Symmetric minimally entangled typical thermal states},
journal = {Phys. Rev. B},
year = {2015},
month = {6},
day = {12},
volume = {92},
number = {115105},
abstract = {We extend White's minimally entangled typically thermal states approach (METTS) to allow Abelian and non-Ablian symmetries to be exploited when computing finite-temperature response functions in one-dimensional (1D) quantum systems. Our approach, called SYMETTS, starts from a METTS sample of states that are not symmetry eigenstates, and generates from each a symmetry eigenstate. These symmetry states are then used to calculate dynamic response functions. SYMETTS is ideally suited to determine the low-temperature spectra of 1D quantum systems with high resolution. We employ this method to study a generalized diamond chain model for the natural mineral azurite Cu3(CO3)2(OH)2, which features a plateau at 13 in the magnetization curve at low temperatures. Our calculations provide new insight into the effects of temperature on magnetization and excitation spectra in the plateau phase, which can be fully understood in terms of the microscopic model.},
DOI = {10.1103/PhysRevB.92.115105},
author = {Bruognolo, Benedikt and von Delft, Jan and Weichselbaum, Andreas}
}
@Article { FukuharaHZSBEG2015,
title = {Crystallization in Ising quantum magnets},
journal = {Science},
year = {2015},
month = {3},
day = {27},
volume = {347},
number = {6229},
pages = {1455-1458},
abstract = {Dominating finite-range interactions in many-body systems can lead to intriguing self-ordered phases of matter. For quantum magnets, Ising models with power-law interactions are among the most elementary systems that support such phases. These models can be implemented by laser coupling ensembles of ultracold atoms to Rydberg states. Here, we report on the experimental preparation of crystalline ground states of such spin systems. We observe a magnetization staircase as a function of the system size and show directly the emergence of crystalline states with vanishing susceptibility. Our results demonstrate the precise control of Rydberg many-body systems and may enable future studies of phase transitions and quantum correlations in interacting quantum magnets.},
DOI = {10.1126/science.1258351},
author = {Schauss, P. and Zeiher, J. and Fukuhara, T. and Hild, S. and Cheneau, M. and Macri, T. and Pohl, T. and Bloch, I. and Gross, C.}
}
@Article { Abdi2015,
title = {Quantum State Engineering with Circuit Electromechanical Three-Body Interactions},
journal = {Phys. Rev. Lett.},
year = {2015},
volume = {114},
pages = {173602},
abstract = {We propose a hybrid system with quantum mechanical three-body interactions between photons, phonons, and qubit excitations. These interactions take place in a circuit quantum electrodynamical architecture with a superconducting microwave resonator coupled to a transmon qubit whose shunt capacitance is free to mechanically oscillate. We show that this system design features a three-mode polariton--mechanical mode and a nonlinear transmon--mechanical mode interaction in the strong coupling regime. Together with the strong resonator--transmon interaction, these properties provide intriguing opportunities for manipulations of this hybrid quantum system. We show, in particular, the feasibility of cooling the mechanical motion down to its ground state and preparing various nonclassical states including mechanical Fock and cat states and hybrid tripartite entangled states.},
DOI = {10.1103/PhysRevLett.114.173602},
author = {Abdi, Mehdi and Pernpeintner, Matthias and Gross, Rudolf and Huebl, Hans and J. Hartmann, Michael}
}
@Article { Caneva2015,
title = {Quantum dynamics of propagating photons with strong interactions: a generalized input-output formalism},
year = {2015},
abstract = {There has been rapid development of systems that yield strong interactions between freely propagating photons in one dimension via controlled coupling to quantum emitters. This raises interesting possibilities such as quantum information processing with photons or quantum many-body states of light, but treating such systems generally remains a difficult task theoretically. Here, we describe a novel technique in which the dynamics and correlations of a few photons can be exactly calculated, based upon knowledge of the initial photonic state and the solution of the reduced effective dynamics of the quantum emitters alone. We show that this generalized ''input-output'' formalism allows for a straightforward numerical implementation regardless of system details, such as emitter positions, external driving, and level structure. As a specific example, we apply our technique to show how atomic systems with infinite-range interactions and under conditions of electromagnetically induced transparency enable the selective transmission of correlated multi-photon states.},
web_url = {http://arxiv.org/abs/1501.04427},
author = {Caneva, Tommaso and T. Manzoni, Marco and Shi, Tao and S. Douglas, James and Ignacio Cirac, J. and E. Chang, Darrick}
}
@Article { Cubitt2015,
title = {Undecidability of the Spectral Gap (short version)},
year = {2015},
abstract = {The spectral gap -- the difference in energy between the ground state and the first excited state -- is one of the most important properties of a quantum many-body system. Quantum phase transitions occur when the spectral gap vanishes and the system becomes critical. Much of physics is concerned with understanding the phase diagrams of quantum systems, and some of the most challenging and long-standing open problems in theoretical physics concern the spectral gap, such as the Haldane conjecture that the Heisenberg chain is gapped for integer spin, proving existence of a gapped topological spin liquid phase, or the Yang-Mills gap conjecture (one of the Millennium Prize problems). These problems are all particular cases of the general spectral gap problem: Given a quantum many-body Hamiltonian, is the system it describes gapped or gapless? Here we show that this problem is undecidable, in the same sense as the Halting Problem was proven to be undecidable by Turing. A consequence of this is that the spectral gap of certain quantum many-body Hamiltonians is not determined by the axioms of mathematics, much as Goedels incompleteness theorem implies that certain theorems are mathematically unprovable. We extend these results to prove undecidability of other low temperature properties, such as correlation functions. The proof hinges on simple quantum many-body models that exhibit highly unusual physics in the thermodynamic limit.},
author = {Cubitt, Toby and Perez-Garcia, David and M. Wolf, Michael}
}
@Article { Cui2015,
title = {Variational matrix product operators for the steady state of dissipative quantum systems},
year = {2015},
abstract = {We present a new variational method, based on the matrix product operator (MPO) ansatz, for finding the steady state of dissipative quantum chains governed by master equations of the Lindblad form. Instead of requiring an accurate representation of the system evolution until the stationary state is attained, the algorithm directly targets the final state, thus allowing for a faster convergence when the steady state is a MPO with small bond dimension. Our numerical simulations for several dissipative spin models over a wide range of parameters illustrate the performance of the method and show that indeed the stationary state is often well described by a MPO of very moderate dimensions.},
author = {Cui, Jian and Ignacio Cirac, J. and Carmen Ba, Mari}
}
@Article { Erhard2015a,
title = {Optical control of internal electric fields in band-gap graded InGaN nanowires},
journal = {Nano Lett.},
year = {2015},
volume = {15},
number = {1},
pages = {332–338},
abstract = {InGaN nanowires are suitable building blocks for many future optoelectronic devices. We show that a linear grading of the indium content along the nanowire axis from GaN to InN introduces an internal electric field evoking a photocurrent. Consistent with quantitative band structure simulations we observe a sign change in the measured photocurrent as a function of photon flux. This negative differential photocurrent opens the path to a new type of nanowire-based photodetector. We demonstrate that the photocurrent response of the nanowires is as fast as 1.5 ps.},
web_url = {http://pubs.acs.org/doi/abs/10.1021/nl503616w},
author = {Erhard, N. and T. M. Golam Sarwar, A. and Yang, F. and W. McComb, D. and C. Myers, R. and W. Holleitner, A.}
}
@Article { Erhard2015,
title = {Ultrafast photocurrents and THz generation in single InAs-nanowires},
year = {2015},
abstract = {To clarify the ultrafast temporal interplay of the different photocurrent mechanisms occurring in single InAs-nanowire-based circuits, an on-chip photocurrent pump-probe spectroscopy based on coplanar striplines was utilized. The data are interpreted in terms of a photo-thermoelectric current and the transport of photogenerated holes to the electrodes as the dominating ultrafast photocurrent contributions. Moreover, it is shown that THz radiation is generated in the optically excited InAs-nanowires, which is interpreted in terms of a dominating photo-Dember effect. The results are relevant for nanowire-based optoelectronic and photovoltaic applications as well as for the design of nanowire-based THz sources.},
web_url = {http://arxiv.org/abs/1502.03782},
author = {Erhard, Nadine and Seifert, Paul and Prechtel, Leonhard and Hertenberger, Simon and Karl, Helmut and Abstreiter, Gerhard and Koblm{\"u}ller, Gregor and W. Holleitner, Alexander}
}
@Article { Flassig2015,
title = {Towards on-chip generation, routing and detection of non-classical light},
journal = {Proc. SPIE 9373},
year = {2015},
abstract = {We fabricate an integrated photonic circuit with emitter, waveguide and detector on one chip, based on a hybrid superconductor-semiconductor system. We detect photoluminescence from self-assembled InGaAs quantum dots on-chip using NbN superconducting nanowire single photon detectors. Using the fast temporal response of these detectors we perform time-resolved studies of non-resonantly excited quantum dots. By introducing a temporal ?ltering to the signal, we are able to resonantly excite the quantum dot and detect its resonance uorescence on-chip with the integrated superconducting single photon detector.},
web_url = {http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2190947},
author = {Flassig, Fabian and Kaniber, Michael and Reithmaier, G and M, Kai and Andrejew, Alexander and Gross, Rudolf and Vu, Jelena and Finley, Jonathan}
}
@Article { Haegeman2015,
title = {Quantum Gross-Pitaevskii Equation},
year = {2015},
abstract = {We introduce a non-commutative generalization of the Gross-Pitaevskii equation for one-dimensional quantum field theories. This generalization is obtained by applying the Dirac-Frenkel time-dependent variational principle to the variational manifold of continuous matrix product states. This allows for a full quantum description of the many body system including entanglement and correlations and thus extends significantly beyond the usual mean-field description of the Gross-Pitaevskii equation, which is known to fail for one-dimensional systems.},
web_url = {http://arxiv.org/abs/1501.06575},
author = {Haegeman, Jutho and Draxler, Damian and Stojevic, Vid and Ignacio Cirac, J. and J. Osborne, Tobias and Verstraete, Frank}
}
@Article { Huegel2015,
title = {Thermodynamics of the Bose-Hubbard model in a Bogoliubov+U theory},
year = {2015},
abstract = {We derive the Bogoliubov+U formalism to study the thermodynamical properties of the Bose-Hubbard model. The framework can be viewed as the zero-frequency limit of bosonic dynamical mean-field theory (B-DMFT), but equally well as an extension of the mean-field decoupling approximation in which pair creation and annihilation of depleted particles is taken into account. The self-energy on the impurity site is treated variationally, minimizing the grand potential. The theory containing just 3 parameters that are determined self-consistently reproduces the T=0 phase diagrams of the 3d and 2d Bose-Hubbard model with an accuracy of 1 \% or better. The superfluid to normal transition at finite temperature is also reproduced well and only slightly less accurately than in B-DMFT.},
web_url = {http://arxiv.org/abs/1501.07849},
author = {H, Dario and Pollet, Lode}
}
@Article { Kastl2015,
title = {Ultrafast helicity control of surface currents in topological insulators with near-unity fidelity},
journal = {Nature Communications},
year = {2015},
volume = {6},
pages = {6617},
abstract = {In recent years, a class of solid state materials, called three-dimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spin-polarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions yet at cryogenic temperatures. Here, we show that surface currents in Bi2Se3 can be controlled by circularly polarized light on a picosecond time scale with a fidelity near unity even at room temperature. We re-veal the temporal separation of such ultrafast helicity-dependent surface currents from photo-induced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators.},
DOI = {10.1038/ncomms7617},
author = {Kastl, Christoph and Karnetzky, Christoph and Karl, Helmut and W. Holleitner, Alexander}
}
@Article { Mueller-Hermes2015,
title = {Positivity of linear maps under tensor powers},
year = {2015},
abstract = {We investigate linear maps between matrix algebras that remain positive under tensor powers, i.e., under tensoring with n copies of themselves. Completely positive and completely co-positive maps are trivial examples of this kind. We show that for every n\(\in\)\textbackslashmathbb{N} there exist non-trivial maps with this property and that for two-dimensional Hilbert spaces there is no non-trivial map for which this holds for all n. For higher dimensions we reduce the existence question of such non-trivial ''tensor-stable positive maps'' to a one-parameter family of maps and show that an affirmative answer would imply the existence of NPPT bound entanglement. As an application we show that any tensor-stable positive map that is not completely positive yields an upper bound on the quantum channel capacity, which for the transposition map gives the well-known cb-norm bound. We furthermore show that the latter is an upper bound even for the LOCC-assisted quantum capacity, and that moreover it is a strong converse rate for this task.},
web_url = {http://arxiv.org/abs/1502.05630},
author = {M, Alexander and Reeb, David and M. Wolf, Michael}
}
@Article { Schraml2015,
title = {Linear and Non-linear Response of Lithographically Defined Plasmonic Nanoantennas},
journal = {Proceedings of SPIE 9371, 93711D (2015)},
year = {2015},
abstract = {We present numerical studies, nano-fabrication and optical characterization of bowtie nanoantennas demonstrating their superior performance with respect to the electric field enhancement as compared to other Au nanoparticle shapes. For optimized parameters, we found mean intensity enhancement factors >2300x in the feed-gap of the antenna, decreasing to 1300x when introducing a 5nm titanium adhesion layer. Using electron beam lithography we fabricated gold bowties on various substrates with feed-gaps and tip radii as small as 10nm. In polarization resolved measurement we experimentally observed a blue shift of the surface plasmon resonance from 1.72eV to 1.35eV combined with a strong modification of the electric field enhancement in the feed-gap. Under excitation with a 100fs pulsed laser source, we observed non-linear light emission arising from two-photon photoluminescence and second harmonic generation from the gold. The bowtie nanoantenna shows a high potential for outstanding conversion efficiencies and the enhancement of other optical effects which could be exploited in future nanophotonic devices.},
DOI = {10.1117/12.2079104},
author = {Schraml, K. and Kaniber, M. and Bartl, J. and Glashagen, G. and Regler, A. and Campbell, T. and J. Finley, J.}
}
@Article { Schreiber2015,
title = {Observation of many-body localization of interacting fermions in a quasi-random optical lattice},
year = {2015},
abstract = {We experimentally observe many-body localization of interacting fermions in a one-dimensional quasi-random optical lattice. We identify the many-body localization transition through the relaxation dynamics of an initially-prepared charge density wave. For sufficiently weak disorder the time evolution appears ergodic and thermalizing, erasing all remnants of the initial order. In contrast, above a critical disorder strength a significant portion of the initial ordering persists, thereby serving as an effective order parameter for localization. The stationary density wave order and the critical disorder value show a distinctive dependence on the interaction strength, in agreement with numerical simulations. We connect this dependence to the ubiquitous logarithmic growth of entanglement entropy characterizing the generic many-body localized phase.},
web_url = {http://arxiv.org/abs/1501.05661},
author = {Schreiber, Michael and S. Hodgman, Sean and Bordia, Pranjal and P. L, Henrik and H. Fischer, Mark and Vosk, Ronen and Altman, Ehud and Schneider, Ulrich and Bloch, Immanuel}
}
@Article { deVegaB2015,
title = {Thermofield-based chain mapping approach for open quantum systems},
year = {2015},
abstract = {We consider a thermofield approach to analyze the evolution of an open quantum system coupled to an environment at finite temperature. In this approach, the finite temperature environment is exactly mapped onto two virtual environments at zero temperature. These two environments are then unitarily transformed into two different chains of oscillators, leading to a one dimensional structure that can be numerically studied using tensor network techniques.},
web_url = {http://arxiv.org/abs/1504.07228},
author = {de Vega, Ines and Ba{\~n}uls, Mari-Carmen}
}
@Article { piazza2013a,
title = {Umklapp Superradiance from a Collisionless Quantum Degenerate Fermi Gas},
journal = {Phys. Rev. Lett.},
year = {2014},
month = {4},
day = {8},
volume = {112,},
pages = {143003},
abstract = {The quantum dynamics of the electromagnetic light mode of an optical cavity filled with a coherently driven Fermi gas of ultracold atoms strongly depends on geometry of the Fermi surface. Superradiant light generation and self-organization of the atoms can be achieved at low pumping threshold due to resonant atom-photon Umklapp processes, where the fermions are scattered from one side of the Fermi surface to the other by exchanging photon momenta. The cavity spectrum exhibits sidebands, that, despite strong atom-light coupling and cavity decay, retain narrow linewidth, due to absorptionless transparency windows outside the atomic particle-hole continuum and the suppression of inhomogeneous broadening and thermal fluctuations in the collisionless Fermi gas.},
web_url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.143003},
DOI = {10.1103/PhysRevLett.112.143003},
author = {Piazza, Francesco and Strack, Philipp}
}
@Article { kuhlen2013,
title = {Unambiguous determination of spin dephasing times in ZnO},
journal = {Phys. Status Solidi B},
year = {2014},
month = {4},
day = {3},
volume = {251,},
number = {9},
pages = {1861},
abstract = {Time-resolved magneto-optics is a well-established optical pump probe technique to generate and to probe spin coherence in semiconductors. By this method, spin dephasing times T_2\verb=^=* can easily be determined if their values are comparable to the available pump-probe-delays. If T_2\verb=^=* exceeds the laser repetition time, however, resonant spin amplification (RSA) can equally be used to extract T_2\verb=^=*. We demonstrate that in ZnO these techniques have several tripping hazards resulting in deceptive values for T_2\verb=^=* and show how to avoid them. We show that the temperature dependence of the amplitude ratio of two separate spin species can easily be misinterpreted as a strongly temperature dependent T_2\verb=^=* of a single spin ensemble, while the two spin species have T_2\verb=^=* values which are nearly independent of temperature. Additionally, consecutive pump pulses can significantly diminish the spin polarization, which remains from previous pump pulses. While this barely affects T_2\verb=^=* values extracted from delay line scans, it results in seemingly shorter T_2\verb=^=* values in RSA.},
web_url = {http://onlinelibrary.wiley.com/doi/10.1002/pssb.201350201/abstract;jsessionid=0E7964788D941B5E670C8F6C297B537B.f01t01},
DOI = {10.1002/pssb.201350201},
author = {Kuhlen, Sebastian and Ledesch, Ralph and de Winter, Robin and Althammer, Matthias and T. B. Goennenwein, Sebastian and Opel, Matthias and Gross, Rudolf and A. Wassner, Thomas and S. Brandt, Martin and Beschoten, Bernd}
}
@Article { aizenman2014,
title = {Resonances and Partial Delocalization on the Complete Graph},
year = {2014},
abstract = {Random operators may acquire extended states formed from a multitude of mutually resonating local quasi-modes. This mechanics is explored here in the context of the random Schr{\"o}dinger operator on the complete graph. The operators exhibits local quasi modes mixed through a single channel. While most of its spectrum consists of localized eigenfunctions, under appropriate conditions it includes also bands of states which are delocalized in the \textbackslashell\verb=^=1-though not in \textbackslashell\verb=^=2-sense, where the eigenvalues have the statistics of \textbackslashv{S}eba spectra. The analysis proceeds through some general observations on the scaling limits of random functions in the Herglotz-Pick class. The results are in agreement with a heuristic condition for the emergence of resonant delocalization, which is stated in terms of the tunneling amplitude among quasi-modes.},
web_url = {http://arxiv.org/abs/1405.3951},
author = {Aizenman, Michael and Shamis, Mira and Warzel, Simone}
}
@Article { buesser2014,
title = {Decoherence of an entangled state of a strongly-correlated double quantum dot structure through tunneling processes},
year = {2014},
abstract = {We consider two quantum dots described by the Anderson-impurity model with one electron per dot. The goal of our work is to study the decay of a maximally entangled state between the two electrons localized in the dots. We prepare the system in a perfect singlet and then tunnel-couple one of the dots to leads, which induces the non-equilibrium dynamics. We identify two cases: if the leads are subject to a sufficiently large voltage and thus a finite current, then direct tunneling processes cause decoherence and the entanglement as well as spin correlations decay exponentially fast. At zero voltage or small voltages and beyond the mixed-valence regime, virtual tunneling processes dominate and lead to a slower loss of coherence. We analyze this problem by studying the real-time dynamics of the spin correlations and the concurrence using two techniques, namely the time-dependent density matrix renormalization group method and a master-equation method. The results from these two approaches are in excellent agreement in the direct-tunneling regime for the case in which the dot is weakly tunnel-coupled to the leads. We present a quantitative analysis of the decay rates of the spin correlations and the concurrence as a function of tunneling rate, interaction strength, and voltage.},
web_url = {http://arxiv.org/abs/1406.4093},
author = {A. B{\"u}sser, C. and de Vega, I. and Heidrich-Meisner, F.}
}
@Article { bukov2014,
title = {Mean-field phase diagram of the Bose-Fermi Hubbard model},
journal = {Phys. Rev. B},
year = {2014},
volume = {89},
pages = {094502},
abstract = {We analyze the ground-state properties of mixtures consisting of scalar bosons and spin-12 fermions using a mean-field treatment of the local boson-fermion interaction on a simple cubic lattice. In the deep superfluid limit of the boson sector and the BCS regime of the fermion sector, we derive BCS-type equations to determine the phase diagram of the system. We find a competition between a charge density wave and a superconducting phase. In the opposite limit, we study the Mott-insulator-to-superfluid transition of the boson sector in the presence of a staggered density-induced alternating potential provided by the fermions, and determine the mean-field transition line. In the two-superfluids phase of the mixture, we restrict to nearest-neighbor-induced interactions between the fermions and consider the extended Hubbard model. We perform a mean-field analysis of the critical temperature for the formation of boson-assisted s-, extended s−-, d-, and p-wave pairs at fermionic half-filling. We compare our results with a recent dynamical mean-field study [P. Anders et al., Phys. Rev. Lett. 109, 206401 (2012)].},
web_url = {http://link.aps.org/doi/10.1103/PhysRevB.89.094502},
publisher = {American Physical Society},
DOI = {10.1103/PhysRevB.89.094502},
author = {Bukov, Marin and Pollet, Lode}
}
@Article { kun2014,
title = {Universal Conductivity in a Two-Dimensional Superfluid-to-Insulator Quantum Critical System},
journal = {Phys. Rev. Lett.},
year = {2014},
volume = {112},
pages = {030402},
abstract = {We compute the universal conductivity of the (2+1)-dimensional XY universality class, which is realized for a superfluid-to-Mott insulator quantum phase transition at constant density. Based on large-scale Monte Carlo simulations of the classical (2+1)-dimensional J-current model and the two-dimensional Bose-Hubbard model, we can precisely determine the conductivity on the quantum critical plateau, \(\sigma\)(\(\infty\))=0.359(4)\(\sigma\)Q with \(\sigma\)Q the conductivity quantum. The universal conductivity curve is the standard example with the lowest number of components where the bottoms-up AdS/CFT correspondence from string theory can be tested and made to use [R. C. Myers, S. Sachdev, and A. Singh, Phys. Rev. D 83, 066017 (2011)]. For the first time, the shape of the \(\sigma\)(i\(\omega\)n)−\(\sigma\)(\(\infty\)) function in the Matsubara representation is accurate enough for a conclusive comparison and establishes the particlelike nature of charge transport. We find that the holographic gauge-gravity duality theory for transport properties can be made compatible with the data if temperature of the horizon of the black brane is different from the temperature of the conformal field theory. The requirements for measuring the universal conductivity in a cold gas experiment are also determined by our calculation.},
web_url = {http://link.aps.org/doi/10.1103/PhysRevLett.112.030402},
publisher = {American Physical Society},
DOI = {10.1103/PhysRevLett.112.030402},
author = {Chen, Kun and Liu, Longxiang and Deng, Youjin and Pollet, Lode and Prokof'ev, Nikolay}
}
@Article { cuenin2014,
title = {Dipoles in Graphene Have Infinitely Many Bound States},
year = {2014},
abstract = {We show that in graphene charge distributions with non-vanishing dipole moment have infinitely many bound states. The corresponding eigenvalues accumulate at the edges of the gap faster than any power.},
web_url = {http://arxiv.org/abs/1403.7160},
author = {Cuenin, Jean-Claude and Siedentop, Heinz}
}
@Article { duca2014,
title = {An Aharonov-Bohm interferometer for determining Bloch band topology},
year = {2014},
abstract = {The geometric structure of an energy band in a solid is fundamental for a wide range of many-body phenomena in condensed matter and is uniquely characterized by the distribution of Berry curvature over the Brillouin zone. In analogy to an Aharonov-Bohm interferometer that measures the magnetic flux penetrating a given area in real space, we realize an atomic interferometer to measure Berry flux in momentum space. We demonstrate the interferometer for a graphene-type hexagonal lattice, where it has allowed us to directly detect the singular \(\pi\) Berry flux localized at each Dirac point. We show that the interferometer enables one to determine the distribution of Berry curvature with high momentum resolution. Our work forms the basis for a general framework to fully characterize topological band structures and can also facilitate holonomic quantum computing through controlled exploitation of the geometry of Hilbert space.},
web_url = {http://arxiv.org/abs/1407.5635},
author = {Duca, Lucia and Li, Tracy and Reitter, Martin and Bloch, Immanuel and Schleier-Smith, Monika and Schneider, Ulrich}
}
@Article { Dumas2014,
title = {Polynomial cubic differentials and convex polygons in the projective plane},
year = {2014},
abstract = {We construct and study a natural homeomorphism between the moduli space of polynomial cubic differentials of degree d on the complex plane and the space of projective equivalence classes of oriented convex polygons with d+3 vertices. This map arises from the construction of a complete hyperbolic affine sphere with prescribed Pick differential, and can be seen as an analogue of the Labourie-Loftin parameterization of convex RP\verb=^=2 structures on a compact surface by the bundle of holomorphic cubic differentials over Teichmuller space.},
web_url = {arxiv.org/abs/1407.8149},
author = {Dumas, David and Wolf, Michael}
}
@Article { fauser2014,
title = {Multiparticle localization for disordered systems on continuous space via the fractional moment method},
year = {2014},
abstract = {We investigate spectral and dynamical localization of a quantum system of n particles on \textbackslashmathbb{R}\verb=^=d which are subject to a random potential and interact through a pair potential which may have infinite range. We establish two conditions which ensure spectral and dynamical localization near the bottom of the spectrum of the n -particle system: i)localization is established in the regime of weak interactions supposing one-particle localization, and ii)localization is also established under a Lifshitz-tail type condition on the sparsity of the spectrum. In case of polynomially decaying interactions, we provide an upper bound on the number of particles up to which these conditions apply.},
web_url = {http://arxiv.org/abs/1402.5832},
author = {Fauser, Michael and Warzel, Simone}
}
@Article { fukuda2014,
title = {Quantum channels with polytopic images and image additivity},
year = {2014},
abstract = {We study quantum channels with respect to their image, i.e., the image of the set of density operators under the action of the channel. We first characterize the set of quantum channels having polytopic images and show that additivity of the minimal output entropy can be violated in this class. We then provide a complete characterization of quantum channels T that are universally image additive in the sense that for any quantum channel S, the image of T \textbackslashotimes S is the convex hull of the tensor product of the images of T and S. These channels turn out to form a strict subset of entanglement breaking channels with polytopic images and a strict superset of classical-quantum channels.},
web_url = {http://arxiv.org/abs/1408.2340},
author = {Fukuda, Motohisa and Nechita, Ion and M. Wolf, Michael}
}
@Article { gebert2014,
title = {The exponent in the orthogonality catastrophe for Fermi gases},
year = {2014},
abstract = {We quantify the asymptotic vanishing of the ground-state overlap of two non-interacting Fermi gases in d-dimensional Euclidean space in the thermodynamic limit. Given two one-particle Schr{\"o}dinger operators in finite-volume which differ by a compactly supported bounded potential, we prove a power-law upper bound on the ground-state overlap of the corresponding non-interacting N-particle systems. We interpret the decay exponent \(\gamma\) in terms of scattering theory and find \(\gamma\) = \(\pi\)\(^{-2}\)\{\textbackslashlVert\textbackslasharcsin{\textbackslashlvert T_E/2\textbackslashrvert\}\textbackslashrVert}_\{\textbackslashmathrm{HS\}}\verb=^=2, where T_E is the transition matrix at the Fermi energy E. This exponent reduces to the one predicted by Anderson [Phys. Rev. 164, 352--359 (1967)] for the exact asymptotics in the special case of a point-like perturbation. We therefore expect the upper bound to coincide with the exact asymptotics of the overlap.},
web_url = {http://arxiv.org/abs/1407.2512},
author = {Gebert, Martin and K{\"u}ttler, Heinrich and M{\"u}ller, Peter and Otte, Peter}
}
@Article { germinet2014,
title = {Ergodicity and dynamical localization for Delone-Anderson operators},
year = {2014},
abstract = {We study the ergodic properties of Delone-Anderson operators, using the framework of randomly coloured Delone sets and Delone dynamical systems. In particular, we show the existence of the integrated density of states and, under some assumptions on the geometric complexity of the underlying Delone sets, we obtain information on the almost-sure spectrum of the family of random operators. We then exploit these results to study the Lifshitz-tail behaviour of the integrated density of states of a Delone-Anderson operator at the bottom of the spectrum. This is used as an input for the multi scale analysis to prove dynamical localization. We also estimate the size of the spectral region where dynamical localization occurs.},
web_url = {http://arxiv.org/abs/1405.4233},
author = {Germinet, Francois and M{\"u}ller, Peter and Rojas-Molina, Constanza}
}
@Article { glasser2014,
title = {Construction of spin models displaying quantum criticality from quantum field theory},
journal = {Nuclear Physics B},
year = {2014},
volume = {886,},
pages = {63-74},
abstract = {We provide a method for constructing finite temperature states of one-dimensional spin chains displaying quantum criticality. These models are constructed using correlators of products of quantum fields and have an analytical purification. Their properties can be investigated by Monte-Carlo simulations, which enable us to study the low-temperature phase diagram and to show that it displays a region of quantum criticality. The mixed states obtained are shown to be close to the thermal state of a simple nearest neighbour Hamiltonian.},
web_url = {http://arxiv.org/abs/1405.4135},
DOI = {10.1016/j.nuclphysb.2014.06.016},
author = {Glasser, Ivan and Ignacio Cirac, J. and Sierra, German and E. B. Nielsen, Anne}
}
@Article { goulko2014,
title = {The effect of spin-orbit interactions on the 0.7-anomaly in quantum point contacts},
year = {2014},
abstract = {We study how the conductance of a quantum point contact is affected by spin-orbit interactions, for systems at zero temperature both with and without electron-electron interactions. In the presence of spin-orbit coupling, tuning the strength and direction of an external magnetic field can change the dispersion relation and hence the local density of states in the point contact region. This modifies the effect of electron-electron interactions, implying striking changes in the shape of the 0.7-anomaly and introducing additional distinctive features in the first conductance step.},
web_url = {http://arxiv.org/abs/1408.0746},
author = {Goulko, Olga and Bauer, Florian and Heyder, Jan and von Delft, Jan}
}
@Article { hanl2014,
title = {Equilibrium Fermi-liquid coefficients for the fully screened N-channel Kondo model},
journal = {Phys. Rev. B},
year = {2014},
volume = {89,},
pages = {195131},
abstract = {We analytically and numerically compute three equilibrium Fermi-liquid coefficients of the fully screened N-channel Kondo model, namely c_B, c_T and c_\textbackslashvarepsilon, characterizing the magnetic field and temperature dependence of the resisitivity, and the curvature of the equilibrium Kondo resonance, respectively. We present a compact, unified derivation of the N-dependence of these coefficients, combining elements from various previous treatments of this model. We numerically compute these coefficients using the numerical renormalization group, with non-Abelian symmetries implemented explicitly, finding agreement with Fermi-liquid predictions on the order of 5\% or better.},
web_url = {http://arxiv.org/abs/1403.0497},
DOI = {10.1103/PhysRevB.89.195131},
author = {Hanl, Markus and Weichselbaum, Andreas and von Delft, Jan and Kiselev, Mikhail}
}
@Article { moritz2014,
title = {Low dimensionality of the surface conductivity of diamond},
journal = {Phys. Rev. B},
year = {2014},
volume = {89},
pages = {115426},
abstract = {Undoped diamond, a remarkable bulk electrical insulator, exhibits a high surface conductivity in air when the surface is hydrogen terminated. Although theoretical models have claimed that a two-dimensional hole gas is established as a result of surface energy-band bending, no definitive experimental demonstration has been reported so far. Here, we prove the two-dimensional character of the surface conductivity by low-temperature characterization of diamond in-plane gated field-effect transistors that enable the lateral confinement of the transistor's drain-source channel to nanometer dimensions. In these devices, we observe Coulomb blockade effects of multiple quantum islands varying in size with the gate voltage. The charging energy and thus the size of these zero-dimensional islands exhibit a gate-voltage dependence which is the direct result of the two-dimensional character of the conductive channel formed at hydrogen-terminated diamond surfaces.},
web_url = {http://link.aps.org/doi/10.1103/PhysRevB.89.115426},
publisher = {American Physical Society},
DOI = {10.1103/PhysRevB.89.115426},
author = {Hauf, Moritz V. and Simon, Patrick and Seifert, Max and Holleitner, Alexander W. and Stutzmann, Martin and Garrido, Jose A.}
}
@Article { hild2014,
title = {Far-from-equilibrium spin transport in Heisenberg quantum magnets},
year = {2014},
abstract = {We study experimentally the far-from-equilibrium dynamics in ferromagnetic Heisenberg quantum magnets realized with ultracold atoms in an optical lattice. After controlled imprinting of a spin spiral pattern with adjustable wave vector, we measure the decay of the initial spin correlations through single-site resolved detection. On the experimentally accessible timescale of several exchange times we find a profound dependence of the decay rate on the wave vector. In one-dimensional systems we observe diffusion-like spin transport with a dimensionless diffusion coefficient of 0.22(1). We show how this behavior emerges from the microscopic properties of the closed quantum system. In contrast to the one-dimensional case, our transport measurements for two-dimensional Heisenberg systems indicate anomalous super-diffusion.},
web_url = {http://arxiv.org/abs/1407.6934},
author = {Hild, Sebastian and Fukuhara, Takeshi and Schauss, Peter and Zeiher, Johannes and Knap, Michael and Demler, Eugene and Bloch, Immanuel and Gross, Christian}
}
@Article { hochstaettler2014,
title = {Semicircle law for a matrix ensemble with dependent entries},
year = {2014},
abstract = {We study ensembles of random symmetric matrices whose entries exhibit certain correlations. Examples are distributions of Curie-Weiss-type. We provide a criterion on the correlations ensuring the validity of Wigner's semicircle law for the eigenvalue distribution measure. In case of Curie-Weiss distributions this criterion applies above the critical temperature (i.e. \(\beta\)<1). We also investigate the largest eigenvalue of certain ensembles of Curie-Weiss type and find a transition in its behavior at the critical temperature.},
web_url = {http://arxiv.org/abs/1401.6636},
author = {Hochst{\"a}ttler, Winfried and Kirsch, Werner and Warzel, Simone}
}
@Article { hocke2014,
title = {Determination of effective mechanical properties of a double-layer beam by means of a nano-electromechanical transducer},
year = {2014},
abstract = {We investigate the mechanical properties of a doubly-clamped, double-layer nanobeam embedded into an electromechanical system. The nanobeam consists of a highly pre-stressed silicon nitride and a superconducting niobium layer. By measuring the mechanical displacement spectral density both in the linear and the nonlinear Duffing regime, we determine the pre-stress and the effective Young's modulus of the nanobeam. An analytical double-layer model quantitatively corroborates the measured values. This suggests that this model can be used to design mechanical multilayer systems for electro- and optomechanical devices, including materials controllable by external parameters such as piezoelectric, magnetrostrictive, or in more general multiferroic materials.},
web_url = {http://arxiv.org/abs/1407.6867},
author = {Hocke, Fredrik and Pernpeintner, Matthias and Zhou, Xiaoqing and Schliesser, Albert and J. Kippenberg, Tobias and Huebl, Hans and Gross, Rudolf}
}
@Article { idel2014,
title = {Sinkhorn normal form for unitary matrices},
year = {2014},
abstract = {Sinkhorn proved that every entry-wise positive matrix can be made doubly stochastic by multiplying with two diagonal matrices. In this note we prove a recently conjectured analogue for unitary matrices: every unitary can be decomposed into two diagonal unitaries and one whose row- and column sums are equal to one. The proof is non-constructive and based on a reformulation in terms of symplectic topology. As a corollary, we obtain a decomposition of unitary matrices into an interlaced product of unitary diagonal matrices and discrete Fourier transformations. This provides a new decomposition of linear optics arrays into phase shifters and canonical multiports described by Fourier transformations.},
web_url = {http://arxiv.org/abs/1408.5728},
author = {Idel, Martin and M. Wolf, Michael}
}
@Article { Keller2014,
title = {An invitation to trees of finite cone type: random and deterministic operators},
year = {2014},
abstract = {Trees of finite cone type have appeared in various contexts. In particular, they come up as simplified models of regular tessellations of the hyperbolic plane. The spectral theory of the associated Laplacians can thus be seen as induced by geometry. Here we give an introduction focusing on background and then turn to recent results for (random) perturbations of trees of finite cone type and their spectral theory.},
web_url = {http://arxiv.org/abs/1403.4426},
author = {Keller, Matthias and Lenz, Daniel and Warzel, Simone}
}
@Article { keyl2014,
title = {Controlling several atoms in a cavity},
journal = {New Journal of Physics},
year = {2014},
volume = {16},
number = {6},
pages = {065010},
abstract = {We treat control of several two-level atoms interacting with one mode of the electromagnetic field in a cavity. This provides a useful model to study pertinent aspects of quantum control in infinite dimensions via the emergence of infinite-dimensional system algebras. Hence we address problems arising with infinite-dimensional Lie algebras and those of unbounded operators. For the models considered, these problems can be solved by splitting the set of control Hamiltonians into two subsets: the first obeys an Abelian symmetry and can be treated in terms of infinite-dimensional Lie algebras and strongly closed subgroups of the unitary group of the system Hilbert space. The second breaks this symmetry, and its discussion introduces new arguments. Yet, full controllability can be achieved in a strong sense: e.g., in a time dependent Jaynes Cummings model we show that, by tuning coupling constants appropriately, every unitary of the coupled system (atoms and cavity) can be approximated with arbitrarily small error.},
web_url = {http://stacks.iop.org/1367-2630/16/i=6/a=065010},
author = {Keyl, Michael and Zeier, Robert and Schulte Herbr{\"u}ggen, Thomas}
}
@Article { klein2014,
title = {Ac-conductivity and electromagnetic energy absorption for the Anderson model in linear response theory},
year = {2014},
abstract = {We continue our study of the ac-conductivity in linear response theory for the Anderson model using the conductivity measure. We establish further properties of the conductivity measure, including nontriviality at nonzero temperature, the high temperature limit, and asymptotics with respect to the disorder. We also calculate the electromagnetic energy absorption in linear response theory in terms of the conductivity measure.},
web_url = {http://arxiv.org/abs/1403.0286},
author = {Klein, Abel and M{\"u}ller, Peter}
}
@Article { kumar2014,
title = {Few-cycle, Broadband, Mid-infrared Optical Parametric Oscillator Pumped by a 20-fs Ti:sapphire Laser},
year = {2014},
abstract = {We report a few-cycle, broadband, singly-resonant optical parametric oscillator (OPO) for the mid-infrared based on MgO-doped periodically-poled LiNbO3 (MgO:PPLN), synchronously pumped by a 20-fs Ti:sapphire laser. By using crystal interaction lengths as short as 250 um, and careful dispersion management of input pump pulses and the OPO resonator, near-transform-limited, few-cycle idler pulses tunable across the mid-infrared have been generated, with as few as 3.7 optical cycles at 2682 nm. The OPO can be continuously tuned over 2179-3732 nm by cavity delay tuning, providing up to 33 mW of output power at 3723 nm. The idler spectra exhibit stable broadband profiles with bandwidths spaning over 422 nm (FWHM) recorded at 3732 nm. We investigate the effect of crystal length on spectral bandwidth and pulse duration at a fixed wavelength, confirming near-transform-limited idler pulses for all grating interaction lengths. By locking the repetition frequency of the pump laser to a radio-frequency reference, and without active stabilization of the OPO cavity length, an idler power stability better than 1.6\% rms over >2.75 hours is obtained when operating at maximum output power, in excellent spatial beam quality with TEM00 mode profile.},
web_url = {http://arxiv.org/abs/1404.1893},
author = {Chaitanya Kumar, Suddapalli and Esteban-Martin, Adolfo and Ideguchi, Takuro and Yan, Ming and Holzner, Simon and W. Haensch, Theodor and Picque, Nathalie and Ebrahim-Zadeh, Majid}
}
@Article { liu2014,
title = {Simplex valence-bond crystal in the spin-1 kagome Heisenberg antiferromagnet},
year = {2014},
abstract = {We investigate the ground state properties of a spin-1 kagome antiferromagnetic Heisenberg (KAH) model using tensor-network methods. We find a trimerized ground state, with energy per site e_0\textbackslashsimeq-1.409 obtained by accurate calculations directly in the thermodynamic limit. The symmetry between left and right triangles is spontaneously broken, with a relative energy difference of \(\delta\) \(\approx\) 20\%. The spin-spin, dimer-dimer, and chiral correlation functions are found to decay exponentially with a rather short correlation length, showing that the ground state is gapped. Based on this unambiguous numerical evidence, we identify the ground state of the spin-1 KAH model to be a simplex valence-bond crystal (SVBC). Besides the KAH model, we also discuss the spin-1 bilinear-biquadratic Heisenberg model on a kagome lattice, and determine its ground state phase diagram. In particular, we find a quantum phase transition between the SVBC and ferro-quadrupolar nematic states.},
web_url = {http://arxiv.org/abs/1406.5905},
author = {Liu, Tao and Li, Wei and Weichselbaum, Andreas and von Delft, Jan and Su, Gang}
}
@Article { lubasch2014,
title = {Algorithms for finite Projected Entangled Pair States},
journal = {Phys. Rev. B},
year = {2014},
volume = {90,},
pages = {064425},
abstract = {Projected Entangled Pair States (PEPS) are a promising ansatz for the study of strongly correlated quantum many-body systems in two dimensions. But due to their high computational cost, developing and improving PEPS algorithms is necessary to make the ansatz widely usable in practice. Here we analyze several algorithmic aspects of the method. On the one hand, we quantify the connection between the correlation length of the PEPS and the accuracy of its approximate contraction, and discuss how purifications can be used in the latter. On the other, we present algorithmic improvements for the update of the tensor that introduce drastic gains in the numerical conditioning and the efficiency of the algorithms. Finally, the state-of-the-art general PEPS code is benchmarked with the Heisenberg and quantum Ising models on lattices of up to 21 \(\times\) 21 sites.},
web_url = {http://arxiv.org/abs/1405.3259},
DOI = {10.1103/PhysRevB.90.064425},
author = {Lubasch, Michael and Ignacio Cirac, J. and Ba{\~n}uls, Mari-Carmen}
}
@Article { :/content/aip/journal/apl/105/8/10.1063/1.4894239,
title = {A carrier relaxation bottleneck probed in single InGaAs quantum dots using integrated superconducting single photon detectors},
journal = {Applied Physics Letters},
year = {2014},
volume = {105},
number = {8},
pages = {-},
abstract = {Using integrated superconducting single photon detectors, we probe ultra-slow exciton capture and relaxation dynamics in single self-assembled InGaAs quantum dots embedded in a GaAs ridge waveguide. Time-resolved luminescence measurements performed with on- and off-chip detection reveal a continuous decrease in the carrier relaxation time from 1.22 ± 0.07 ns to 0.10 ± 0.07 ns upon increasing the number of non-resonantly injected carriers. By comparing off-chip time-resolved spectroscopy with spectrally integrated on-chip measurements, we identify the observed dynamics in the rise time (\(\tau\)r ) as arising from a relaxation bottleneck at low excitation levels. From the comparison with the temporal dynamics of the single exciton transition with the on-chip emission signal, we conclude that the relaxation bottleneck is circumvented by the presence of charge carriers occupying states in the bulk material and the two-dimensional wetting layer continuum. A characteristic \(\tau\)r \(\propto\) P −2∕3 power law dependence is observed suggesting Auger-type scattering between carriers trapped in the quantum dot and the two-dimensional wetting layer continuum which circumvents the phonon relaxation bottleneck.},
web_url = {http://scitation.aip.org/content/aip/journal/apl/105/8/10.1063/1.4894239},
DOI = {http://dx.doi.org/10.1063/1.4894239},
author = {Reithmaier, G. and Flassig, F. and Hasch, P. and Lichtmannecker, S. and M{\"u}ller, K. and Vučković, J. and Gross, R. and Kaniber, M. and Finley, J. J.}
}
@Article { PSSR:PSSR201308305,
title = {Polarization dependent, surface plasmon induced photoconductance in gold nanorod arrays},
journal = {physica status solidi (RRL) – Rapid Research Letters},
year = {2014},
volume = {8},
number = {3},
pages = {264--268},
abstract = {We report on the photoconductance in two-dimensional arrays of gold nanorods. The arrays are formed by a combination of droplet deposition and stamping methods. We find that the plasmon induced photoconductance is sensitive to the linear polarization of the exciting photons consistent with the excitation of the longitudinal surface plasmon resonance of the nanorods.},
keywords = {surface plasmons, optical sensors, nanorods, photoconductance, gold},
web_url = {http://dx.doi.org/10.1002/pssr.201308305},
publisher = {WILEY-VCH Verlag},
ISSN = {1862-6270},
DOI = {10.1002/pssr.201308305},
author = {Diefenbach, S. and Erhard, N. and Schopka, J. and Martin, A. and Karnetzky, C. and Iacopino, D. and Holleitner, A. W.}
}
@Article { PhysRevApplied.2.024002,
title = {Optical Thermometry of an Electron Reservoir Coupled to a Single Quantum Dot in the Millikelvin Range},
journal = {Phys. Rev. Applied},
year = {2014},
volume = {2},
pages = {024002},
abstract = {We show how resonant laser spectroscopy of the trion optical transitions in a self-assembled quantum dot can be used to determine the temperature of a nearby electron reservoir. At finite magnetic field, the spin-state occupation of the Zeeman-split quantum-dot electron ground states is governed by thermalization with the electron reservoir via cotunneling. With resonant spectroscopy of the corresponding excited trion states, we map out the spin occupation as a function of magnetic field to establish optical thermometry for the electron reservoir. We demonstrate the implementation of the technique in the subkelvin temperature range where it is most sensitive and where the electron temperature is not necessarily given by the cryostat base temperature.},
url = {http://arxiv.org/abs/1405.2261},
web_url = {http://link.aps.org/doi/10.1103/PhysRevApplied.2.024002},
publisher = {American Physical Society},
DOI = {10.1103/PhysRevApplied.2.024002},
author = {Seilmeier, F. and Hauck, M. and Schubert, E. and Schinner, G. J. and Beavan, S. E. and H{\"o}gele, A.}
}
@Article { seilmeier2014,
title = {Sub-Kelvin optical thermometry of an electron reservoir coupled to a self-assembled InGaAs quantum dot},
journal = {Phys. Rev. Applied},
year = {2014},
volume = {2,},
pages = {024002},
abstract = {We show how resonant laser spectroscopy of the trion optical transitions in a self-assembled quantum dot can be used to determine the temperature of a nearby electron reservoir. At finite magnetic field the spin-state occupation of the Zeeman-split quantum dot electron ground states is governed by thermalization with the electron reservoir via co-tunneling. With resonant spectroscopy of the corresponding excited trion states we map out the spin occupation as a function of magnetic field to establish optical thermometry for the electron reservoir. We demonstrate the implementation of the technique in the sub-Kelvin temperature range where it is most sensitive, and where the electron temperature is not necessarily given by the cryostat base temperature.},
author = {Seilmeier, F. and Hauck, M. and Schubert, E. and J. Schinner, G. and E. Beavan, S. and H, A.}
}
@Article { PhysRevB.90.235306,
title = {Locating environmental charge impurities with confluent laser spectroscopy of multiple quantum dots},
journal = {Phys. Rev. B},
year = {2014},
volume = {90},
pages = {235306},
web_url = {http://link.aps.org/doi/10.1103/PhysRevB.90.235306},
publisher = {American Physical Society},
DOI = {10.1103/PhysRevB.90.235306},
author = {Hauck, M. and Seilmeier, F. and Beavan, S. E. and Badolato, A. and Petroff, P. M. and H{\"o}gele, A.}
}
@Article { Adame2014,
title = {Exponential vanishing of the ground-state gap of the QREM via adiabatic quantum computing},
year = {2014},
abstract = {In this note we compile and slightly generalise ideas of Farhi, Goldstone, Gosset, Gutmann, Nagaj and Shor by discussing a lower bound on the run time of their quantum adiabatic search algorithm and its use for an upper bound on the energy gap above the ground-state of the generators of this algorithm. We illustrate these ideas by applying them to the quantum random energy model (QREM). Our main result is a simple proof of the conjectured exponential vanishing of the energy gap of the QREM.},
author = {Adame, Juan and Warzel, Simone}
}
@Article { Bruognolo2014,
title = {Two-bath spin-boson model: Phase diagram and critical properties},
journal = {Phys. Rev. B},
year = {2014},
volume = {90,},
pages = {245130},
abstract = {The spin-boson model, describing a two-level system coupled to a bath of harmonic oscillators, is a generic model for quantum dissipation, with manifold applications. It has also been studied as a simple example for an impurity quantum phase transition. Here we present a detailed study of a U(1)-symmetric two-bath spin-boson model, where two different components of an SU(2) spin 1/2 are coupled to separate dissipative baths. Non-trivial physics arises from the competition of the two dissipation channels, resulting in a variety of phases and quantum phase transitions. We employ a combination of analytical and numerical techniques to determine the properties of both the stable phases and the quantum critical points. In particular, we find a critical intermediate-coupling phase which is bounded by a continuous quantum phase transition which violates the quantum-to-classical correspondence.},
author = {Bruognolo, Benedikt and Weichselbaum, Andreas and Guo, Cheng and von Delft, Jan and Schneider, Imke and Vojta, Matthias}
}
@Article { Cai2014,
title = {Identifying a bath-induced Bose liquid in interacting spin-boson models},
journal = {Phys. Rev. Lett.},
year = {2014},
volume = {113,},
pages = {260403},
abstract = {We study the ground state phase diagram of a one-dimensional hard-core bosonic model with nearest-neighbor interactions (XXZ model) where every site is coupled Ohmically to an independent but identical reservoir, hereby generalizing spin-boson models to interacting spin-boson systems. We show that a bath-induced Bose metal phase can occur in the ground state phase diagram away from half filling. This phase is compressible, gapless, and conducting but not superfluid. At haf-filling, only a Luttinger liquid and a charge density wave are found. The phase transition between them is of Kosterlitz-Thouless type where the Luttinger parameter takes a non-universal value.The applied quantum Monte Carlo method can be used for all open bosonic and unfrustrated spin systems, regardless of their dimension, filling factor and spectrum of the dissipation as long as the quantum system couples to the bath via the density operators.},
author = {Cai, Zi and Schollw{\"o}ck, Ulrich and Pollet, Lode}
}
@Article { Ehberger2014,
title = {Highly coherent electron beam from a laser-triggered tungsten needle tip},
year = {2014},
abstract = {We report on a quantitative measurement of the spatial coherence of electrons emitted from a sharp metal needle tip. We investigate the coherence in photoemission using near-ultraviolet laser triggering with a photon energy of 3.1 eV and compare it to DC-field emission. A carbon-nanotube is brought in close proximity to the emitter tip to act as an electrostatic biprism. From the resulting electron matter wave interference fringes we deduce an upper limit of the effective source radius both in laser-triggered and DC-field emission mode, which quantifies the spatial coherence of the emitted electron beam. We obtain (0.80\textbackslashpm 0.05)\textbackslash,nm in laser-triggered and (0.55\textbackslashpm 0.02)\textbackslash,nm in DC-field emission mode, revealing that the outstanding coherence properties of electron beams from needle tip field emitters are largely maintained in laser-induced emission. In addition, the relative coherence width of 0.36 of the photoemitted electron beam is the largest observed so far. The preservation of electronic coherence during emission as well as ramifications for time-resolved electron imaging techniques are discussed.},
author = {Ehberger, Dominik and Hammer, Jakob and Eisele, Max and Kr, Michael and Noe, Jonathan and H, Alexander and Hommelhoff, Peter}
}
@Article { Glaetzle2014,
title = {Frustrated Quantum Magnetism with Laser-Dressed Rydberg Atoms},
journal = {Phys. Rev. Lett.},
year = {2014},
volume = {114},
number = {173002},
abstract = {We show how a broad class of lattice spin-1/2 models with angular- and distance-dependent couplings can be realized with cold alkali atoms stored in optical or magnetic trap arrays. The effective spin-1/2 is represented by a pair of atomic ground states, and spin-spin interactions are obtained by admixing van der Waals interactions between fine-structure split Rydberg states with laser light. The strengths of the diagonal spin interactions as well as the ''flip-flop'', and ''flip-flip'' and ''flop-flop'' interactions can be tuned by exploiting quantum interference, thus realizing different spin symmetries. The resulting energy scales of interactions compare well with typical temperatures and decoherence time-scales, making the exploration of exotic forms of quantum magnetism, including emergent gauge theories and compass models, accessible within state-of-the-art experiments.},
DOI = {10.1103/PhysRevLett.114.173002},
author = {W. Glaetzle, Alexander and Dalmonte, Marcello and Nath, Rejish and Gross, Christian and Bloch, Immanuel and Zoller, Peter}
}
@Article { Gloeckner2014,
title = {Rotational state detection of electrically trapped polyatomic molecules},
year = {2014},
abstract = {Detecting the internal state of polar molecules is a substantial challenge when standard techniques such as resonance-enhanced multi photon ionization (REMPI) or laser-induced fluorescense (LIF) do not work. As this is the case for most polyatomic molecule species, we here investigate an alternative based on state selective removal of molecules from an electrically trapped ensemble. Specifically, we deplete molecules by driving rotational and/or vibrational transitions to untrapped states. Fully resolving the rotational state with this method can be a considerable challenge as the frequency differences between various transitions is easily substantially less than the Stark broadening in an electric trap. However, making use of a unique trap design providing homogeneous fields in a large fraction of the trap volume, we successfully discriminate all rotational quantum numbers, including the rotational M-substate.},
author = {Gl, Rosa and Prehn, Alexander and Rempe, Gerhard and Zeppenfeld, Martin}
}
@Article { Handrek2014,
title = {The Ground State Energy of Heavy Atoms: the Leading Correction},
year = {2014},
abstract = {For heavy atoms (large atomic number Z) described by no-pair operators in the Furry picture we find the ground state's leading energy correction. We compare the result with (semi-)empirical values and Schwinger's prediction showing more than qualitative agreement.},
author = {Handrek, Michael and Siedentop, Heinz}
}
@Article { Knips2014,
title = {Multipartite entanglement dectection with minimal effort},
year = {2014},
abstract = {Certifying entanglement in a multipartite state is a demanding task. A state of N qubits is parametrized by 4\verb=^=N-1 real numbers, so, at first glimpse, one may expect that the measurement complexity of generic entanglement detection is also exponential with N. However, here we show how to design indicators for genuine multipartite quantum entanglement which require only two correlation measurements for prominent quantum states. We introduce a constructive method to derive such criteria and apply them in experiments for four-qubit Greenberger-Horne-Zeilinger states, cluster states and Dicke states.},
author = {Knips, Lukas and Schwemmer, Christian and Klein, Nico and Wie?niak, Marcin and Weinfurter, Harald}
}
@Article { Kroiss2014,
title = {Diagrammatic Monte Carlo study of mass-imbalanced Fermi-polaron system},
year = {2014},
abstract = {We apply the diagrammatic Monte Carlo approach to three-dimensional Fermi-polaron systems with mass-imbalance, where an impurity interacts resonantly with a noninteracting Fermi sea whose atoms have a different mass. This method allows to go beyond frequently used variational techniques by stochastically summing all relevant impurity Feynman diagrams up to a maximum expansion order limited by the sign problem. Polaron energy and quasiparticle residue can be accurately determined over a broad range of impurity masses. Furthermore, the spectral function of an imbalanced polaron demonstrates the stability of the quasiparticle and allows to locate in addition also the repulsive polaron as an excited state. The quantitative exactness of two-particle-hole wave-functions is investigated, resulting in a relative lowering of polaronic energies in the mass-imbalance phase diagram. Tan's contact coefficient for the mass-balanced polaron system is found in good agreement with variational methods. Mass-imbalanced systems can be studied experimentally by ultracold atom mixtures like \verb=^=6Li-\(^{40}\)K.},
author = {Kroiss, Peter and Pollet, Lode}
}
@Article { Raczkowski2014,
title = {Spin and charge dynamics of a quasi-one-dimensional antiferromagnetic metal},
journal = {Phys. Rev. B},
year = {2014},
volume = {91,},
pages = {045137},
abstract = {We use quantum Monte Carlo simulations to study a finite-temperature dimensional-crossover-driven evolution of spin and charge dynamics in weakly coupled Hubbard chains with a half-filled band. The low-temperature behavior of the charge gap indicates a crossover between two distinct energy scales: a high-energy one-dimensional (1D) Mott gap due to the umklapp process and a low-energy gap which stems from long-range antiferromagnetic (AF) fluctuations. Away from the 1D regime and at temperature scales above the charge gap, the emergence of a zero-frequency Drude-like feature in the interchain optical conductivity \textbackslashsigma\(_{\textbackslashperp}\)(\(\omega\)) implies the onset of a higher-dimensional metal. In this metallic phase, enhanced quasiparticle scattering off finite-range AF fluctuations results in incoherent single-particle dynamics. The coupling between spin and charge fluctuations is also seen in the spin dynamical structure factor S({\textbackslashpmb q},\(\omega\)) displaying damped spin excitations (paramagnons) close to the AF wave-vector {\textbackslashpmb q}=(\(\pi\),\(\pi\)) and particle-hole continua near 1D momentum transfers spanning quasiparticles at the Fermi surface. We relate our results to the charge deconfinement in quasi-1D organic Bechgaard-Fabre salts.},
author = {Raczkowski, Marcin and F. Assaad, Fakher and Pollet, Lode}
}
@Article { Reiserer2014a,
title = {A Quantum Gate between a Flying Optical Photon and a Single Trapped Atom},
journal = {Nature},
year = {2014},
volume = {508,},
pages = {237-240},
abstract = {The steady increase in control over individual quantum systems has backed the dream of a quantum technology that provides functionalities beyond any classical device. Two particularly promising applications have been explored during the past decade: First, photon-based quantum communication, which guarantees unbreakable encryption but still has to be scaled to high rates over large distances. Second, quantum computation, which will fundamentally enhance computability if it can be scaled to a large number of quantum bits. It was realized early on that a hybrid system of light and matter qubits could solve the scalability problem of both fields - that of communication via quantum repeaters, that of computation via an optical interconnect between smaller quantum processors. To this end, the development of a robust two-qubit gate that allows to link distant computational nodes is ''a pressing challenge''. Here we demonstrate such a quantum gate between the spin state of a single trapped atom and the polarization state of an optical photon contained in a faint laser pulse. The presented gate mechanism is deterministic, robust and expected to be applicable to almost any matter qubit. It is based on reflecting the photonic qubit from a cavity that provides strong light-matter coupling. To demonstrate its versatility, we use the quantum gate to create atom-photon, atom-photon-photon, and photon-photon entangled states from separable input states. We expect our experiment to break ground for various applications, including the generation of atomic and photonic cluster states, Schr{\"o}dinger-cat states, deterministic photonic Bell-state measurements, and quantum communication using a redundant quantum parity code.},
author = {Reiserer, Andreas and Kalb, Norbert and Rempe, Gerhard and Ritter, Stephan}
}
@Article { Reiserer2014,
title = {Cavity-based quantum networks with single atoms and optical photons},
year = {2014},
abstract = {The implementation of a large-scale quantum network is a key challenge for quantum science. Such network consists of stationary quantum nodes that can store and process quantum information locally. The nodes are connected by quantum channels for flying information carriers, i.e. photons. These channels serve both to directly exchange quantum information between nodes as well as to distribute entanglement over the whole network. In order to scale such network to many particles and long distances, an efficient interface between the nodes and the channels is required. This article describes the cavity-based approach to this goal, with an emphasis on experimental systems in which single atoms are trapped in and coupled to optical resonators. Besides being conceptually appealing, this approach is promising for quantum networks on larger scales, as it gives access to long qubit coherence times and high light-matter coupling efficiencies. Thus, it allows one to generate entangled photons on the push of a button, to reversibly map the quantum state of a photon onto an atom, to transfer and teleport quantum states between remote atoms, to entangle distant atoms, to detect optical photons nondestructively, to perform entangling quantum gates between an atom and one or several photons, and even provides a route towards efficient heralded quantum memories for future repeaters. The presented general protocols and the identification of key parameters are applicable to other experimental systems.},
author = {Reiserer, Andreas and Rempe, Gerhard}
}
@Article { Schwemmer2014,
title = {Experimental multipartite entanglement without multipartite correlations},
year = {2014},
abstract = {Non-classical correlations between measurement results make entanglement the essence of quantum physics and the main resource for quantum information applications. Surprisingly, there are n-particle states which do not exhibit n-partite correlations at all but still are genuinely n-partite entangled. We introduce a general construction principle for such states, implement them in a multiphoton experiment and analyze their properties in detail. Remarkably, even without n-partite correlations, these states do violate Bell inequalities showing that there is no local realistic model describing their properties.},
author = {Schwemmer, Christian and Knips, Lukas and Cong Tran, Minh and de Rosier, Anna and Laskowski, Wieslaw and Paterek, Tomasz and Weinfurter, Harald}
}
@Article { Schwemmer2014a,
title = {Experimental Comparison of Efficient Tomography Schemes for a Six-Qubit State},
journal = {Phys. Rev. Lett.},
year = {2014},
volume = {113,},
pages = {040503},
abstract = {Quantum state tomography suffers from the measurement effort increasing exponentially with the number of qubits. Here, we demonstrate permutationally invariant tomography for which, contrary to conventional tomography, all resources scale polynomially with the number of qubits both in terms of the measurement effort as well as the computational power needed to process and store the recorded data. We demonstrate the benefits of combining permutationally invariant tomography with compressed sensing by studying the influence of the pump power on the noise present in a six-qubit symmetric Dicke state, a case where full tomography is possible only for very high pump powers.},
author = {Schwemmer, Christian and Toth, Geza and Niggebaum, Alexander and Moroder, Tobias and Gross, David and G, Otfried and Weinfurter, Harald}
}
@Article { Tiarks2014,
title = {Single-Photon Transistor Using a F{\"o}rster Resonance},
journal = {Phys. Rev. Lett.},
year = {2014},
volume = {113,},
pages = {053602},
abstract = {An all-optical transistor is a device in which a gate light pulse switches the transmission of a target light pulse with a gain above unity. The gain quantifies the change of the transmitted target photon number per incoming gate photon. We study the quantum limit of one incoming gate photon and observe a gain of 20. The gate pulse is stored as a Rydberg excitation in an ultracold gas. The transmission of the subsequent target pulse is suppressed by Rydberg blockade which is enhanced by a F{\"o}rster resonance. The detected target photons reveal in a single shot with a fidelity above 0.86 whether a Rydberg excitation was created during the gate pulse. The gain offers the possibility to distribute the transistor output to the inputs of many transistors, thus making complex computational tasks possible.},
author = {Tiarks, Daniel and Baur, Simon and Schneider, Katharina and D, Stephan and Rempe, Gerhard}
}
@Article { GebertKM2014,
title = {Anderson's orthogonality catastrophe},
journal = {Commun. Math. Phys.},
year = {2014},
volume = {329},
pages = {979-998},
abstract = {We give an upper bound on the modulus of the ground-state overlap of two non-interacting fermionic quantum systems with N particles in a large but finite volume Ld of d-dimensional Euclidean space. The underlying one-particle Hamiltonians of the two systems are standard Schr{\"o}dinger operators that differ by a non-negative compactly supported scalar potential. In the thermodynamic limit, the bound exhibits an asymptotic power-law decay in the system size L, showing that the ground-state overlap vanishes for macroscopic systems. The decay exponent can be interpreted in terms of the total scattering cross section averaged over all incident directions. The result confirms and generalises P. W. Anderson's informal computation [Phys. Rev. Lett. 18, 1049--1051 (1967)].},
DOI = {10.1007/s00220-014-1914-3},
author = {Gebert, Martin and K{\"u}ttler, Heinrich and M{\"u}ller, Peter}
}
@Article { piazza2013,
title = {Bose-Einstein Condensation versus Dicke-Hepp-Lieb Transition in an Optical Cavity},
journal = {Annals of Physics},
year = {2013},
month = {12},
volume = {339,},
pages = {135},
abstract = {We provide an exact solution for the interplay between Bose-Einstein condensation and the Dicke-Hepp-Lieb self-organization transition of an ideal Bose gas trapped inside a single-mode optical cavity and subject to a transverse laser drive. Based on an effective action approach, we determine the full phase diagram at arbitrary temperature, which features a bi-critical point where the transitions cross. We calculate the dynamically generated band structure of the atoms and the associated supression of the critical temperature for Bose-Einstein condensation in the phase with a spontaneous periodic density modulation. Moreover, we determine the evolution of the polariton spectrum due to the coupling of the cavity photons and the atomic field near the self-organization transition, which is quite different above or below the Bose-Einstein condensation temperature. At low temperatures, the critical value of the Dicke-Hepp-Lieb transition decreases with temperature and thus thermal fluctuations can enhance the tendency to a periodic arrangement of the atoms.},
web_url = {https://www.sciencedirect.com/science/article/pii/S0003491613001905?via\%3Dihub},
DOI = {10.1016/j.aop.2013.08.015},
author = {Piazza, Francesco and Strack, Philipp and Zwerger, Wilhelm}
}
@Article { Rath20130,
title = {Field-theoretical study of the Bose polaron},
journal = {Phys. Rev. A},
year = {2013},
month = {11},
day = {26},
volume = {88,},
pages = {053632},
abstract = {We study the properties of the Bose polaron, an impurity strongly interacting with a Bose-Einstein condensate, using a field-theoretic approach and make predictions for the spectral function and various quasiparticle properties that can be tested in experiment. We find that most of the spectral weight is contained in a coherent attractive and a metastable repulsive polaron branch. We show that the qualitative behavior of the Bose polaron is well described by a non-selfconsistent T-matrix approximation by comparing analytical results to numerical data obtained from a fully selfconsistent T-matrix approach. The latter takes into account an infinite number of bosons excited from the condensate.},
web_url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.88.053632},
DOI = {10.1103/PhysRevA.88.053632},
author = {Patrick Rath, Steffen and Schmidt, Richard}
}
@Article { rath2013,
title = {Non-local order in Mott insulators, Duality and Wilson Loops},
journal = {Annals of Physics (N.Y.)},
year = {2013},
month = {7},
volume = {Annals},
pages = {334,256(2013)},
abstract = {It is shown that the Mott insulating and superfluid phases of bosons in an optical lattice may be distinguished by a non-local 'parity order parameter' which is directly accessible via single site resolution imaging. In one dimension, the lattice Bose model is dual to a classical interface roughening problem. We use known exact results from the latter to prove that the parity order parameter exhibits long range order in the Mott insulating phase, consistent with recent experiments by Endres et al. [Science 334, 200 (2011)]. In two spatial dimensions, the parity order parameter can be expressed in terms of an equal time Wilson loop of a non-trivial U(1) gauge theory in 2+1 dimensions which exhibits a transition between a Coulomb and a confining phase. The negative logarithm of the parity order parameter obeys a perimeter law in the Mott insulator and is enhanced by a logarithmic factor in the superfluid.},
web_url = {https://www.sciencedirect.com/science/article/pii/S0003491613000717?via\%3Dihub},
DOI = {10.1016/j.aop.2013.04.006},
author = {Patrick Rath, Steffen and Simeth, Wolfgang and Endres, Manuel and Zwerger, Wilhelm}
}
@Article { handrek2012,
title = {On the Maximal Excess Charge of the Chandrasekhar-Coulomb Hamiltonian in Two Dimensions},
journal = {Letters in Mathematical Physics},
year = {2013},
month = {3},
day = {3},
volume = {103},
number = {8},
pages = {843–849},
abstract = {We show that for the straightforward quantized relativistic Coulomb Hamiltonian of a two-dimensional atom -- or the corresponding magnetic quantum dot -- the maximal number of electrons does not exceed twice the nuclear charge. It result is then generalized to the presence of external magnetic fields and atomic Hamiltonians. This is based on the positivity of |\textbackslashbx| T(\textbackslashbp) + T(\textbackslashbp) |\textbackslashbx| which -- in two dimensions -- is false for the non-relativistic case T(\textbackslashbp) = \textbackslashbp\verb=^=2, but is proven in this paper for T(\textbackslashbp) = |\textbackslashbp|, i.e., the ultra-relativistic kinetic energy.},
web_url = {https://link.springer.com/article/10.1007\%2Fs11005-013-0618-5},
DOI = {10.1007/s11005-013-0618-5},
author = {Handrek, Michael and Siedentop, Heinz}
}
@Article { aizenman2011,
title = {Resonant delocalization for random Schr{\"o}dinger operators on tree graphs},
journal = {Journal of the European Mathematical Society (JEMS)},
year = {2013},
volume = {1167-1222 (2013)},
pages = {JournaloftheEuropeanMathematicalSociety(JEMS),15(4), 1167-1222(2013)},
abstract = {We analyse the spectral phase diagram of Schr{\"o}dinger operators T +\(\lambda\) V on regular tree graphs, with T the graph adjacency operator and V a random potential given by iid random variables. The main result is a criterion for the emergence of absolutely continuous (ac) spectrum due to fluctuation-enabled resonances between distant sites. Using it we prove that for unbounded random potentials ac spectrum appears at arbitrarily weak disorder (\(\lambda\) {\ll} 1) in an energy regime which extends beyond the spectrum of T. Incorporating considerations of the Green function's large deviations we obtain an extension of the criterion which indicates that, under a yet unproven regularity condition of the large deviations' 'free energy function', the regime of pure ac spectrum is complementary to that of previously proven localization. For bounded potentials we disprove the existence at weak disorder of a mobility edge beyond which the spectrum is localized.},
web_url = {http://arxiv.org/abs/1104.0969},
DOI = {10.4171/JEMS/389},
author = {Aizenman, Michael and Warzel, Simone}
}
@Article { bauer2013,
title = {Microscopic origin of the 0.7-anomaly in quantum point contacts},
journal = {Nature},
year = {2013},
volume = {501},
pages = {73-78},
abstract = {Quantum point contacts are narrow, one-dimensional constrictions usually patterned in a two-dimensional electron system, for example by applying voltages to local gates. The linear conductance of a point contact, when measured as function of its channel width, is quantized (1, 2, 3) in units of GQ = 2e2/h, where e is the electron charge and h is Planck’s constant. However, the conductance also has an unexpected shoulder at \verb=~=0.7GQ, known as the ‘0.7-anomaly’ (4, 5, 6, 7, 8, 9, 10, 11, 12), whose origin is still subject to debate (11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21). Proposed theoretical explanations have invoked spontaneous spin polarization (4, 17), ferromagnetic spin coupling (19), the formation of a quasi-bound state leading to the Kondo effect (13, 14), Wigner crystallization (16, 20) and various treatments of inelastic scattering18, 21. However, explicit calculations that fully reproduce the various experimental observations in the regime of the 0.7-anomaly, including the zero-bias peak that typically accompanies it6, 9, 10, 11, are still lacking. Here we offer a detailed microscopic explanation for both the 0.7-anomaly and the zero-bias peak: their common origin is a smeared van Hove singularity in the local density of states at the bottom of the lowest one-dimensional subband of the point contact, which causes an anomalous enhancement in the Hartree potential barrier, the magnetic spin susceptibility and the inelastic scattering rate. We find good qualitative agreement between theoretical calculations and experimental results on the dependence of the conductance on gate voltage, magnetic field, temperature, source–drain voltage (including the zero-bias peak) and interaction strength. We also clarify how the low-energy scale governing the 0.7-anomaly depends on gate voltage and interactions. For low energies, we predict and observe Fermi-liquid behaviour similar to that associated with the Kondo effect in quantum dots (22). At high energies, however, the similarities between the 0.7-anomaly and the Kondo effect end.},
DOI = {10.1038/nature12421},
author = {Bauer, F. and Heyder, E. and Schubert and Borowsky, D. and Taubert, D. and Bruognolo, B. and Schuh, S. and Wegscheider, W. and von Delft, J. and Ludwig, S.}
}
@Article { chen2013,
title = {Positivity of |p|\verb=^=a|q|\verb=^=b+|q|\verb=^=b|p|\verb=^=a},
year = {2013},
abstract = {We show that J\(_{a,b,n}\):=\textbackslashfrac12(|p|\verb=^=a|q|\verb=^=b+|q|\verb=^=b|p|\verb=^=a) is positive, if n\textbackslashgeq b+a. (Here q is the multiplication by x and p:= \textbackslashmathrm{i}\(^{-1}\)\textbackslashnabla.) Furthermore we show that it generalizes the generalized Hardy inequalities for the fractional Laplacians.},
web_url = {http://arxiv.org/abs/1301.1524},
author = {Chen, Li and Siedentop, Heinz}
}
@Article { dolde2013,
title = {High fidelity spin entanglement using optimal control},
journal = {Nat. Commun.},
year = {2013},
volume = {5,},
pages = {3371},
abstract = {Precise control of quantum systems is of fundamental importance for quantum device engineering, such as is needed in the fields of quantum information processing, high-resolution spectroscopy and quantum metrology. When scaling up the quantum registers in such devices, several challenges arise: individual addressing of qubits in a dense spectrum while suppressing crosstalk, creation of entanglement between distant nodes, and decoupling from unwanted interactions. The experimental implementation of optimal control is a prerequisite to meeting these challenges. Using engineered microwave pulses, we experimentally demonstrate optimal control of a prototype solid state spin qubit system comprising thirty six energy levels. The spin qubits are associated with proximal nitrogen-vacancy (NV) centers in diamond. We demonstrate precise single-electron spin qubit operations with an unprecedented fidelity F \(\approx\) 0.99 in combination with high-efficiency storage of electron spin states in a nuclear spin quantum memory. Matching single-electron spin operations with spin-echo techniques, we further realize high-quality entangled states (F > 0.82) between two electron spins on demand. After exploiting optimal control, the fidelity is mostly limited by the coherence time and imperfect initialization. Errors from crosstalk in a crowded spectrum of 8 lines as well as detrimental effects from active dipolar couplings have been simultaneously eliminated to unprecedented extent. Finally, by entanglement swapping to nuclear spins, nuclear spin entanglement over a length scale of 25 nm is demonstrated. This experiment underlines the importance of optimal control for scalable room temperature spin-based quantum information devices.},
DOI = {10.1038/ncomms4371},
author = {Dolde, Florian and Bergholm, Ville and Wang, Ya and Jakobi, Ingmar and Pezzagna, Sebastien and Meijer, Jan and Neumann, Philipp and Schulte-Herbrueggen, T. and Biamonte, Jacob and Wrachtrup, J{\"o}rg}
}
@Article { forster2013,
title = {Characterization of Qubit Dephasing by Landau-Zener Interferometry},
journal = {Phys. Rev. Lett.},
year = {2013},
volume = {112,},
pages = {116803},
abstract = {Controlling coherent interaction at avoided crossings is at the heart of quantum information processing. The regime between sudden switches and adiabatic transitions is characterized by quantum superpositions that enable interference experiments. Here, we implement periodic passages at intermediate speed in a GaAs-based two-electron charge qubit and observe Landau-Zener-St{\"u}ckelberg-Majorana (LZSM) quantum interference of the resulting superposition state. We demonstrate that LZSM interferometry is a viable and very general tool to not only study qubit properties but beyond to decipher decoherence caused by complex environmental influences. Our scheme is based on straightforward steady state experiments. The coherence time of our two-electron charge qubit is limited by electron-phonon interaction. It is much longer than previously reported for similar structures.},
web_url = {http://arxiv.org/abs/1309.5907},
DOI = {10.1103/PhysRevLett.112.116803},
author = {Forster, F. and Petersen, G. and Manus, S. and H{\"a}nggi, P. and Schuh, D. and Wegscheider, W. and Kohler, S. and Ludwig, S.}
}
@Article { kendirlik2013,
title = {Anomalous resistance overshoot in the integer quantum Hall effect},
journal = {Nature Sci. Rep},
year = {2013},
volume = {3},
pages = {3133},
abstract = {In this work we report experiments on defined by shallow etching narrow Hall bars. The magneto-transport properties of intermediate mobility two-dimensional electron systems are investigated and analyzed within the screening theory of the integer quantized Hall effect. We observe a non-monotonic increase of Hall resistance at the low magnetic field ends of the quantized plateaus, known as the overshoot effect. Unexpectedly, for Hall bars that are defined by shallow chemical etching the overshoot effect becomes more pronounced at elevated temperatures. We observe the overshoot effect at odd and even integer plateaus, which favor a spin independent explanation, in contrast to discussion in the literature. In a second set of the experiments, we investigate the overshoot effect in gate defined Hall bar and explicitly show that the amplitude of the overshoot effect can be directly controlled by gate voltages. We offer a comprehensive explanation based on scattering between evanescent incompressible channels.},
DOI = {10.1038/srep03133},
author = {M. Kendirlik, E. and Sirt, S. and B. Kalkan, S. and Dietsche, W. and Wegscheider, W. and Ludwig, S. and Siddiki, A.}
}
@Article { liebermeister2013,
title = {Tapered fiber coupling of single photons emitted by a deterministically positioned single nitrogen vacancy center},
journal = {Appl. Phys. Lett.},
year = {2013},
volume = {104,},
pages = {031101},
abstract = {A diamond nano-crystal hosting a single nitrogen vacancy (NV) center is optically selected with a confocal scanning microscope and positioned deterministically onto the subwavelength-diameter waist of a tapered optical fiber (TOF) with the help of an atomic force microscope. Based on this nano-manipulation technique we experimentally demonstrate the evanescent coupling of single fluorescence photons emitted by a single NV-center to the guided mode of the TOF. By comparing photon count rates of the fiber-guided and the free-space modes and with the help of numerical FDTD simulations we determine a lower and upper bound for the coupling efficiency of (9.5+/-0.6)\% and (10.4+/-0.7)\%, respectively. Our results are a promising starting point for future integration of single photon sources into photonic quantum networks and applications in quantum information science.},
web_url = {http://dx.doi.org/10.1063/1.4862207},
author = {Liebermeister, Lars and Petersen, Fabian and v. M{\"u}nchow, Asmus and Burchardt, Daniel and Hermelbracht, Juliane and Tashima, Toshiyuki and W. Schell, Andreas and Benson, Oliver and Meinhardt, Thomas and Krueger, Anke and Stiebeiner, Ariane and Rauschenbeutel, Arno and Weinfurter, Harald and Weber, Markus}
}
@Article { PhysRevLett.110.127403,
title = {Confinement and Interaction of Single Indirect Excitons in a Voltage-Controlled Trap Formed Inside Double InGaAs Quantum Wells},
journal = {Phys. Rev. Lett.},
year = {2013},
volume = {110},
pages = {127403},
abstract = {Voltage-tunable quantum traps confining individual spatially indirect and long-living excitons are realized by providing a coupled double quantum well with nanoscale gates. This enables us to study the transition from confined multiexcitons down to a single, electrostatically trapped indirect exciton. In the few exciton regime, we observe discrete emission lines identified as resulting from a single dipolar exciton, a biexciton, and a triexciton, respectively. Their energetic splitting is well described by Wigner-like molecular structures reflecting the interplay of dipolar interexcitonic repulsion and spatial quantization.},
web_url = {http://link.aps.org/doi/10.1103/PhysRevLett.110.127403},
publisher = {American Physical Society},
DOI = {10.1103/PhysRevLett.110.127403},
author = {Schinner, G. J. and Repp, J. and Schubert, E. and Rai, A. K. and Reuter, D. and Wieck, A. D. and Govorov, A. O. and Holleitner, A. W. and Kotthaus, J. P.}
}
@Article { HofmannGNBDH2013,
title = {Bright, long-lived and coherent excitons in carbon nanotube quantum dots},
journal = {Nat Nano},
year = {2013},
volume = {8},
number = {7},
pages = {502-505},
abstract = {Carbon nanotubes exhibit a wealth of unique physical properties. By virtue of their exceptionally low mass and extreme stiffness they provide ultrahigh-quality mechanical resonances (1), promise long electron spin coherence times in a nuclear-spin free lattice (2, 3) for quantum information processing and spintronics, and feature unprecedented tunability of optical transitions (4, 5) for optoelectronic applications (6). Excitons in semiconducting single-walled carbon nanotubes (7, 8) could facilitate the upconversion of spin (9), mechanical (10) or hybrid spin–mechanical (11) degrees of freedom to optical frequencies for efficient manipulation and detection. However, successful implementation of such schemes with carbon nanotubes has been impeded by rapid exciton decoherence at non-radiative quenching sites (12), environmental dephasing (13) and emission intermittence (14). Here we demonstrate that these limitations may be overcome by exciton localization in suspended carbon nanotubes. For excitons localized in nanotube quantum dots we found narrow optical lines free of spectral wandering, radiative exciton lifetimes (15, 16, 17) and effectively suppressed blinking. Our findings identify the great potential of localized excitons for efficient and spectrally precise interfacing of photons, phonons and spins in novel carbon nanotube-based quantum devices.},
web_url = {http://dx.doi.org/10.1038/nnano.2013.119},
DOI = {10.1038/nnano.2013.119},
author = {Hofmann, Matthias S. and Gluckert, Jan T. and Noe, Jonathan and Bourjau, Christian and Dehmel, Raphael and H{\"o}gele, Alexander}
}
@Article { bermejovega2012,
title = {Classical simulations of Abelian-group normalizer circuits with intermediate measurements},
year = {2012},
abstract = {Quantum normalizer circuits were recently introduced as generalizations of Clifford circuits [arXiv:1201.4867]: a normalizer circuit over a finite Abelian group G is composed of the quantum Fourier transform (QFT) over G, together with gates which compute quadratic functions and automorphisms. In [arXiv:1201.4867] it was shown that every normalizer circuit can be simulated efficiently classically. This result provides a nontrivial example of a family of quantum circuits that cannot yield exponential speed-ups in spite of usage of the QFT, the latter being a central quantum algorithmic primitive. Here we extend the aforementioned result in several ways. Most importantly, we show that normalizer circuits supplemented with intermediate measurements can also be simulated efficiently classically, even when the computation proceeds adaptively. This yields a generalization of the Gottesman-Knill theorem (valid for n-qubit Clifford operations [quant-ph/9705052, quant-ph/9807006] to quantum circuits described by arbitrary finite Abelian groups. Moreover, our simulations are twofold: we present efficient classical algorithms to sample the measurement probability distribution of any adaptive-normalizer computation, as well as to compute the amplitudes of the state vector in every step of it. Finally we develop a generalization of the stabilizer formalism [quant-ph/9705052, quant-ph/9807006] relative to arbitrary finite Abelian groups: for example we characterize how to update stabilizers under generalized Pauli measurements and provide a normal form of the amplitudes of generalized stabilizer states using quadratic functions and subgroup cosets.},
web_url = {http://arxiv.org/abs/1210.3637},
author = {Bermejo-Vega, Juan and Van den Nest, Maarten}
}
@Article { barthel2012,
title = {Scaling of the thermal spectral function for quantum critical bosons in one dimension},
year = {2012},
abstract = {We present an improved scheme for the precise evaluation of finite-temperature response functions of strongly correlated systems in the framework of the time-dependent density matrix renormalization group. The maximum times that we can reach at finite temperatures T are typically increased by a factor of two, when compared against the earlier approaches. This novel scheme, complemented with linear prediction, allows us now to evaluate dynamic correlators for interacting bosons in one dimension. We demonstrate that the considered spectral function in the quantum critical regime with dynamic critical exponent z=2 is captured by the universal scaling form S(k,omega)=(1/T)*Phi(k/sqrt(T),omega/T) and calculate the scaling function precisely.},
author = {Barthel, Thomas and Schollw, Ulrich and Sachdev, Subir}
}
@Article { Urbaszek2012,
title = {Nuclear spin physics in quantum dots: an optical investigation},
journal = {Reviews of Modern Physics},
year = {2012},
volume = {85,},
pages = {79},
abstract = {The mesoscopic spin system formed by the 10E4-10E6 nuclear spins in a semiconductor quantum dot offers a unique setting for the study of many-body spin physics in the condensed matter. The dynamics of this system and its coupling to electron spins is fundamentally different from its bulk counter-part as well as that of atoms due to increased fluctuations that result from reduced dimensions. In recent years, the interest in studying quantum dot nuclear spin systems and their coupling to confined electron spins has been fueled by its direct implication for possible applications of such systems in quantum information processing as well as by the fascinating nonlinear (quantum-)dynamics of the coupled electron-nuclear spin system. In this article, we review experimental work performed over the last decades in studying this mesoscopic,coupled electron-nuclear spin system and discuss how optical addressing of electron spins can be exploited to manipulate and read-out quantum dot nuclei. We discuss how such techniques have been applied in quantum dots to efficiently establish a non-zero mean nuclear spin polarization and, most recently, were used to reduce fluctuations of the average quantum dot nuclear spin orientation. Both results in turn have important implications for the preservation of electron spin coherence in quantum dots, which we discuss. We conclude by speculating how this recently gained understanding of the quantum dot nuclear spin system could in the future enable experimental observation of quantum-mechanical signatures or possible collective behavior of mesoscopic nuclear spin ensembles.},
web_url = {http://arxiv.org/abs/1202.4637},
DOI = {http://dx.doi.org/10.1103/RevModPhys.85.79},
author = {Urbaszek, Bernhard and Marie, Xavier and Amand, Thierry and Krebs, Olivier and Voisin, Paul and Maletinsky, Patrick and H{\"o}gele, Alexander and Imamoglu, Atac}
}
@Article { dallAcqua2011,
title = {Real analyticity away from the nucleus of pseudorelativistic Hartree-Fock orbitals},
journal = {Analysis \& PDE},
year = {2011},
volume = {5},
pages = {,no.3,657--691},
abstract = {We prove that the Hartree--Fock orbitals of pseudorelativistic atoms, that is, atoms where the kinetic energy of the electrons is given by the pseudorelativistic operator sqrt{-Delta+1}-1, are real analytic away from the origin. As a consequence, the quantum mechanical ground state of such atoms is never a Hartree-Fock state. Our proof is inspired by the classical proof of analyticity by nested balls of Morrey and Nirenberg. However, the technique has to be adapted to take care of the non-local pseudodifferential operator, the singularity of the potential at the origin, and the non-linear terms in the equation.},
web_url = {http://arxiv-web3.library.cornell.edu/abs/1103.5026},
author = {Dall'Acqua, Anna and Fournais, S{\o}ren and {\O}stergaard S{\o}rensen, Thomas and Stockmeyer, Edgardo}
}
@Article { barthel2011,
title = {Quasi-locality and efficient simulation of Markovian quantum dynamics},
journal = {Phys. Rev. Lett.},
year = {2011},
volume = {108,},
pages = {230504},
abstract = {We consider open many-body systems governed by a time-dependent quantum master equation with short-range interactions. With a generalized Lieb-Robinson bound, we show that the evolution in this very generic framework is quasi-local, i.e., the evolution of observables can be approximated by implementing the dynamics only in a vicinity of the observables' support. The precision increases exponentially with the diameter of the considered subsystem. Hence, the time-evolution can be simulated on classical computers with a cost that is independent of the system size. Providing error bounds for Trotter decompositions, we conclude that the simulation on a quantum computer is additionally efficient in time. For experiments and simulations, our result can be used to rigorously bound finite-size effects.},
author = {Barthel, Thomas and Kliesch, Martin}
}
@Article { barthel2009,
title = {Spectral functions in one-dimensional quantum systems at T>0},
journal = {Phys. Rev. B},
year = {2009},
volume = {79,},
pages = {245101},
abstract = {We present for the first time time-dependent density-matrix renormalization-group simulations (t-DMRG) at finite temperatures. It is demonstrated how a combination of finite-temperature t-DMRG and time-series prediction allows for an easy and very accurate calculation of spectral functions in one-dimensional quantum systems, irrespective of their statistics, for arbitrary temperatures. This is illustrated with spin structure factors of XX and XXX spin-1/2 chains. For the XX model we can compare against an exact solution and for the XXX model (Heisenberg antiferromagnet) against a Bethe Ansatz solution and quantum Monte Carlo data.},
author = {Barthel, Thomas and Schollw{\"o}ck, Ulrich and R. White, Steven}
}
@Article { bloch2007,
title = {Many-Body Physics with Ultracold Gases},
journal = {Rev. Mod. Phys.},
year = {2007},
volume = {80,},
pages = {885},
abstract = {This article reviews recent experimental and theoretical progress on many-body phenomena in dilute, ultracold gases. Its focus are effects beyond standard weak-coupling descriptions, like the Mott-Hubbard-transition in optical lattices, strongly interacting gases in one and two dimensions or lowest Landau level physics in quasi two-dimensional gases in fast rotation. Strong correlations in fermionic gases are discussed in optical lattices or near Feshbach resonances in the BCS-BEC crossover.},
author = {Bloch, I. and Dalibard, J. and Zwerger, R.}
}