Dynamical Quantum Phase Transitions in Spin Chains with Long-Range Interactions: Merging Different Concepts of Nonequilibrium Criticality
Zunkovic, Bojan, Heyl, Markus, Knap, Michael and Silva, Alessandro
Physical Review Letters , Volume 120, page: 130601
March 2018

Abstract: We theoretically study the dynamics of a transverse-field Ising chain with power-law decaying interactions characterized by an exponent α, 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 α≤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 α and agrees with the order parameter transition for α≤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.

Almost conserved operators in nearly many-body localized systems
Pancotti, Nicola, Knap, Michael, Huse, David A., Cirac, J. Ignacio and Bañuls, Mari-Carmen
Physical Review B , Volume 97, page: 094206
March 2018

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.

Angle-resolved photoemission spectroscopy with quantum gas microscopes
Bohrdt, A., Greif, D., Demler, Eugene, Knap, Michael and Grusdt, F.
Physical Review B , Volume 97(12), page: 125117
March 2018

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.

Ultrafast quantum control of ionization dynamics in krypton
Hütten, Konrad, Mittermair, Michael, Stock, Sebastian O., Beerwerth, Randolf, Shirvanyan, Vahe, Riemensberger, Johann, Duensing, Andreas, Heider, Rupert, Wagner, Martin S., Guggenmos, Alexander, Fritzsche, Stephan, Kabachnik, Nikolay M., Kienberger, Reinhard and Bernhardt, Brigitta
Nature Communications , Volume 9(719)
February 2018

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.

Photon-Mediated Quantum Gate between Two Neutral Atoms in an Optical Cavity
Welte, Stephan, Hacker, Bastian, Daiss, Severin, Ritter, Stephan and Rempe, Gerhard
Physics Review X , Volume 8, page: 011018
February 2018

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  μs. 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.

Spin Hall photoconductance in a three-dimensional topological insulator at room temperature
Seifert, Paul, Vaklinova, Kristina, Ganichev, Sergey, Kern, Klaus, Burghard, Marko and Holleitner, Alexander W.
Nature Communications , Volume 9(331)
January 2018

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.

Universal many-body response of heavy impurities coupled to a Fermi sea: a review of recent progress
Schmidt, Richard, Knap, Michael, Ivanov, Dmitri A, You, Jhih-Shih, Cetina, Marko and Demler, Eugene
Document number: 2
January 2018

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 γ=πkB 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≤ 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.

Exploring 4D quantum Hall physics with a 2D topological charge pump
Lohse, Michael, Schweizer, Christian, Price, Hannah M., Zilberberg, Oded and Bloch, Immanuel
Nature , Volume 553, page: 55-58
January 2018

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.

Size-driven quantum phase transitions
Bausch, Johannes, Cubitt, Toby, Lucia, Angelo, Perez-Garcia, David and Wolf, Michael
Proceedings of the National Academy of Sciences , Volume 115(1), page: 19-23
January 2018

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 15×15, 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.

Bounds on the entanglement entropy of droplet states in the XXZ spin chain
Beaud, V. and Warzel, Simone
Journal of Mathematical Physics , Volume 59, page: 012109
January 2018

Abstract: We consider a class of one-dimensional quantum spin systems on the finite lattice Λ⊂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 Λ = B ∪ 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.

Decoherence-protected memory for a single-photon qubit
Körber, Matthias, Morin, O., Langenfeld, S., Neuzner, A., Ritter, Stephan and Rempe, Gerhard
Nature Photonics , Volume 12, page: 18-21
December 2017

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.

Edge Switching Transformations of Quantum Graphs
Aizenman, Michael, Schanz, H., Smilansky, U. and Warzel, Simone
Acta Physica Polonica A , Volume 132(6), page: 1699-1703
December 2017

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 Schrödinger operator is interlaced with that of the original one. By implication, under edge swap the spectra before and after the transformation, denoted by {Eₙ}^{∞}ₙ₌₁ and {Ẽₙ}^{∞}ₙ₌₁ correspondingly, are level-2 interlaced, so that Eₙ-₂ ≤ Ẽₙ ≤ Eₙ₊₂. The proofs are guided by considerations of the quantum graphs' discrete analogs.

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