Press Releases

"Synopsis: ARPES with Cold Atoms"

This article by Michael Schirber was published in Physics.

 

"The atoms in a gas are normally easier to manipulate than the electrons in a solid, which is why physicists often use ultracold gases as analogs of condensed-matter systems. To probe these ultracold gases, a numerical study proposes using an analog of a condensed-matter experimental technique, called angle-resolved photoemission spectroscopy, or ARPES. If realized, such ARPES-like experiments would provide detailed spectra of the atoms in an ultracold gas. The approach could also provide a new avenue for exploring the physics underlying high-temperature superconductors.

ARPES is a common tool for investigating strongly correlated electrons in solids, such as topological materials and superconductors. The method uses a beam of light to kick out electrons whose momentum and energy are then measured. The spectrum of these photoemitted electrons reveals information about the electronic band structure in the material.

Annabelle Bohrdt from the Technical University of Munich in Germany and her colleagues envisaged ARPES-like measurements in the framework of quantum gas microscopy. The team’s proposed setup is a 1D optical lattice holding a chain of ultracold atoms. A tunable modulation to the lattice’s optical fields excites one atom from this 1D gas, causing it to be kicked into an adjacent “detection” lattice that is initially empty. The authors show how the momentum of this “photoemitted” atom can be measured by essentially letting it rattle around inside the detection lattice. In numerical simulations, the team generated an ARPES-like spectrum by varying the excitation energy for the kicked-out atoms. This technique could potentially be used to study quantum correlations among atoms that ​have been prepared to simulate ​a ​particular condensed-matter model. One such model might be the Fermi-Hubbard model, which is thought to be relevant to superconductivity."

–Michael Schirber

 Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.

 

This research, is published in Physical Review B.

A. Bohrdt, D. Greif, E. Demler, M. Knap, F. Grusdt
Phys. Rev. B 97, 125117 (2018) [arXiv:1710.08925], Editors' suggestion, Featured in Physics

A quadrillionth of a second in slow motion

Many chemical processes run so fast that they are only roughly understood. To clarify these processes, a team from the Technical University of Munich (TUM) has now developed a methodology with a resolution of quintillionths of a second. The new technology stands to help better understand processes like photosynthesis and develop faster computer chips.

An important intermediary step in many chemical processes is ionization. A typical example of this is photosynthesis. The reactions take only a few femtoseconds (quadrillionths of a second) or even merely a few hundred attoseconds (quintillionths of a second). Because they run so extremely fast, only the initial and final products are known, but not the reaction paths or the intermediate products.

To observe such ultrafast processes, science needs a measurement technology that is faster than the observed process itself. So-called “pump-probe spectroscopy” makes this possible.

Here, the sample is excited using an initial laser pulse, which sets the reaction into motion. A second, time-delayed pulse queries the momentary state of the process. Multiple repetitions of the reaction with different time delays result in individual stop-motion images, which can then be compiled into a “film clip”.

Two eyes see more than one

Now, a team of scientists headed by Birgitta Bernhardt, a former staff member at the Chair of Laser and X-ray Physics at TU Munich and meanwhile junior professor at the Institute of Applied Physics at the University of Jena, have for the first time succeeded in combining two pump-probe spectroscopy techniques using the inert gas krypton. This allowed them to shed light on the ultrafast ionization processes in a precision that has simply not been possible hitherto.

“Prior to our experiment, one could observe either which part of the exciting light was absorbed by the sample over time or measure what kind of and how many ions were created in the process,” explains Bernhardt. “We have now combined the two techniques, which allows us to observe the precise steps by which the ionization takes place, how long these intermediate products exist and what precisely the exciting laser pulse causes in the sample.”

Ultrafast processes under control

The combination of the two measuring techniques allows the scientists not only to record the ultrafast ionization processes. Thanks to the variation in the intensity of the second, probing laser pulse, they can now, for the first time, also control and in this way also influence the ionization dynamics.

“This kind of control is a very powerful instrument,” explains Bernhardt. “If we can precisely understand and even influence fast ionization processes, we are able to learn a lot about light-driven processes like photosynthesis – especially about the initial moments in which this complex machinery is set into motion and which is hardly understood to date.”

Ultrafast computers

The technology developed by Bernhardt and her colleagues is also interesting for the development of new, faster computer chips in which the ionization of silicon plays a significant role. If the ionization states of silicon can not only be sampled on such a short time scale, but can also be set – as the first experiments with krypton suggest – scientists might one day be able to use this to develop novel and even faster computer technologies.

Further information:

The work is the result of a collaboration between the workgroups led by Prof. Reinhard Kienberger, who heads the Chair of Laser and X-ray Physics at TU Munich and Stephan Fritzsche, professor at the Institute of Theoretical Physics of the Friedrich Schiller University of Jena.

The research was funded by the European Research Council (ERC), the German Federal Ministry of Education and Research (BMBF), the Max Planck Society, the Max Planck Institute of Quantum Optics, the German Research Foundation (in the context of the Cluster of Excellence Munich Center for Advanced Photonics, MAP), the Alexander von Humboldt Foundation, the Carl Zeiss Foundation, the Donostia International Physics Center of the Donostia-San Sebastián University (Spain) and the workgroup Small Quantum Systems of the European XFEL in Hamburg.

Publication:

Konrad Hütten, Michael Mittermair, Sebastian O. Stock, Randolf Beerwerth, Vahe Shirvanyan, Johann Riemensberger, Andreas Duensing, Rupert Heider, Martin S. Wagner, Alexander Guggenmos, Stephan Fritzsche, Nikolay M. Kabachnik, Reinhard Kienberger and Birgitta Bernhardt.
Ultrafast Quantum Control of Ionization Dynamics in Krypton
Nature Communications, 9, 719 (218) – DOI: 10.1038/s41467-018-03122-1


Contact:

Prof. Dr. Birgitta Bernhardt (Jun.-Prof.)
Friedrich-Schiller-Universität Jena
Abbe Center of Photonics
Albert-Einstein-Straße 6, 07745 Jena, Germany
Tel.: +49 3641 94 7818E-mail

Prof. Dr. Reinhard Kienberger
Technical University of Munich
Chair for Laser and X-ray Physics, E11
James Frank Str., 85748 Garching, Germany
Tel.: +49 89 289 12840E-mail - Internet

 

View the original press release at TUM webpage.

Size-driven quantum phase transitions

A study, published in PNAS [1] and co-authored by MQC member Michael Wolf, reveals potential physical consequences of the recently proven undecidability of the spectral gap [2]. The paper shows the existence of two-dimensional spin-lattices that exhibit a sharp transition from purely classical behavior to an exotic topologically ordered quantum system. The exceptional feature of the constructed spin-models is that this transition is induced by a tiny change of the size of the system and that the scale at which this occurs can be tuned to any desired order of magnitude. These 'size-driven quantum phase transitions' are proven to be thermally stable and involve local interactions that might be within experimental reach.   


[1] J.Bausch, T.S. Cubitt, A.Lucia, D.Perez-Garcia, M.M. Wolf: "Size-driven quantum phase transitions", Proc Natl Acad Sci,115(1):19-23, Jan 2018.
[2] T.S. Cubitt, D. Perez-Garcia, M.M. Wolf: "Undecidability of the spectral gap", Nature, 528(7581):207– 211, Dec 2015.

Light controls two-atom quantum computation

Some powerful rulers of the world may dream of the possibility to get in touch with their colleagues on different continents unnoticed by friends or foes. Someday, new quantum technologies could allow for making these wishes come true. Physicists around the world are working on the realization of large scale quantum networks in which single light quanta transfer (secret) quantum information to stationary nodes at large distances. Such quantum networks’ fundamental building blocks are, for example, quantum repeaters that counteract the loss of quantum information over large distances, or quantum logic gates that are necessary for processing quantum information. 

Now, a team of scientists around Professor Gerhard Rempe, director at the Max Planck Institute of Quantum Optics and head of the Quantum Dynamics Division, has demonstrated the feasibility a new concept for a quantum gate (Phys. Rev. X 8, 011018, 6 February 2018). Here, photons impinging on an optical cavity mediate an interaction between two atoms trapped inside. This interaction is the basis for performing characteristic gate operations between the atoms, for example the operation as a CNOT gate or the generation of entanglement. The new method offers a variety of advantages: for example, the gate operations take place within microseconds which is an asset for quantum information processing. Also, the gate mechanism can be applied to other experimental platforms, and the two-atom gate can serve as a building block in a quantum repeater.

Core element of the experiment (see figure 1) is an asymmetric high-finesse optical resonator, consisting of a high-reflection mirror (left) and a mirror with a finite transmission (right). Two electrically neutral rubidium atoms are trapped in the centre of the cavity. Each atom carries a qubit, i.e. quantum information that is encoded in the superposition of two stable ground states which correspond to the classical bits “0” and “1”. “One of the ground states is in resonance with the cavity’s light field. Hence, atoms and cavity form a strongly coupled system,” Stephan Welte explains, who works on the experiment for his doctoral thesis. “That’s why the atoms can talk to each other. This process cannot take place in free space.”

To execute the gate, single photons are sent onto the semi-transparent mirror. Then, depending on the initial states of the atoms, different scenarios are possible. “When both atoms are in the non-coupling state the photon can enter the cavity, and a standing light wave between the two mirrors builds up,” says Bastian Hacker, another doctoral candidate on the experiment. “The atoms can communicate via this light field: if it is present, the phase of the stored qubits gets rotated by 180 degrees.” In all other cases, if one or both atoms are in resonance with the cavity modes, the photon gets blocked from the cavity, and the state of the atoms does not acquire a phase shift.

These effects are used to execute basic mathematical operations (quantum gates) between the two atoms, as is demonstrated by the Garching team with two characteristic gate operations. On the one hand, the scientists show that their experimental setup can work as a typical C(ontrolled)NOT gate: here the input state of the (control) qubit decides whether the other one’s (target’s) state is being changed or not. In order to demonstrate this functionality, the gate operation is executed on a set of four orthogonal input states, and in each case the resulting output state is determined. From these measurements a table is derived that resembles a classical XOR gate.

On the other hand, in another series of measurements the scientists prove the creation of quantum entangled output states from two initially independent atoms. “To this end, the atoms are prepared in a coherent superposition of both ground states,” Stephan Welte points out. “Therefore, both cases – that the photon enters the cavity and that it is rejected – are quantum-mechanically superimposed, and the gate operation leads to the entanglement of the atoms.”

“The mechanism underlying the gate operation is very simple and elegant because it only comprises one physical step. In contrast to other gate mechanisms the distance between the qubits – in our case 2 to 12 micrometres – does not matter at all,” Bastian Hacker emphasizes. “Also, our gate does not rely on the specific platform of rubidium atoms. It could equally well be applied to many other types of atoms, ions or, for example, solid state quantum dots that carry quantum information.” Professor Gerhard Rempe even envisions further extensions of the system. “We consider placing several atoms, instead of just two, into the cavity. Our gate mechanism could operate on many of them at the same time.” In a large-scale quantum network, multi-qubit nodes could serve as small quantum computers that perform basic computations and send their results to other nodes. 

 

Olivia Meyer-Streng

view original press release on MPQ webpage.

Visit by Prof. Dan Stamper-Kurn

Dan Stamper-Kurn is Professor of Physics at the University of California and a Faculty Scientist at the Lawrence Berkeley National Laboratory, both in California, USA. He is an experimentalist who uses ultracold atomic gases to explore fundamental phenomena in condensed-matter physics, atomic physics, and quantum optics. Prof. Stamper-Kurn also chairs the Science Definition Team of the Bose-Einstein Condensation Cold Atom Laboratory (BECCAL), a joint mission of the German (DLR) and American (NASA) space agencies that aims to operate a quantum gas experiment aboard the International Space Station.

Long-lived storage of a photonic qubit for worldwide teleportation

Concerning the development of quantum memories for the realization of global quantum networks, scientists of the Quantum Dynamics Division led by Professor Gerhard Rempe at the Max Planck Institute of Quantum Optics have now achieved a major breakthrough: they demonstrated the long-lived storage of a photonic qubit on a single atom trapped in an optical resonator. The coherence time of the stored quantum bit outlasts 100 milliseconds and therefore matches the requirement for the creation of a global quantum network in which qubits are directly teleported between end nodes. “The coherence times that we achieve represent an improvement by two orders of magnitude compared to the current state-of-the-art”, says Professor Rempe. (Nature Photonics, 11 December 2017)


Light is an ideal carrier for quantum information encoded on single photons, but transfer over long distances is inefficient and unreliable due to losses. Direct teleportation between the end nodes of a network can be utilized to prevent the loss of precious quantum bits. First, remote entanglement has to be created between the nodes; then, a suitable measurement on the sender side triggers the “spooky action at a distance”, i.e. the instantaneous transport of the qubit to the receiver’s node. However, the quantum bit may be rotated when it reaches the receiver and hence has to be reverted. To this end, the necessary information has to be classically communicated from sender to receiver. This takes a certain amount of time, during which the qubit has to be preserved at the receiver. Considering two network nodes at the most distant places on earth, this corresponds to a time span of 66 milliseconds.

In 2011, Professor Rempe’s group has demonstrated a successful technique for storing a photonic quantum bit on a single atom. The atom is placed in the centre of an optical cavity which is formed by two high-finesse mirrors and hold in place by standing light waves. A single photon which carries the quantum bit in a coherent superposition of two polarization states starts to strongly interact with the single atom once it is sent into the resonator. Ultimately, the photon is absorbed by the atom and the quantum bit is transferred into a coherent superposition of two atomic states. The challenge is to maintain the atomic superposition as long as possible. In former experiments, the storage time was limited to a few hundreds of microseconds.

“The major problem for storing quantum bits is the phenomenon of dephasing,” explains Stefan Langenfeld, a doctoral candidate at the experiment. “Characteristic of a quantum bit is the relative phase of the wave functions of the atomic states that are coherently superimposed. Unfortunately, in real-world experiments, this phase relation is lost over time mostly due to interaction with fluctuating ambient magnetic fields.”

In their current experiment, the scientists take new measures to counteract the impact of those fluctuations. Once the information is transferred from the photon to the atom, the population of one atomic state is coherently transferred to another state. This is done by using a pair of laser beams to induce a Raman transition. In this new configuration, the stored qubit is 500 times less sensitive to magnetic field fluctuations.

Before the retrieval of the stored photonic quantum bit, the Raman transition is reversed. For a storage time of 10 milliseconds, the overlap of the stored photon with the retrieved photon is about 90%. This means, that the mere transfer of the atomic qubit to a less sensitive state configuration extends the coherence time by a factor of 10. Another factor of 10 was gained by adding a so-called “spin echo” to the experimental sequence. Here, the population of the two atomic states used for storage is swapped in the middle of the storage time. “The new technique allows us to preserve the quantum nature of the stored bit for more than 100 milliseconds”, says Matthias Körber, a doctoral candidate at the experiment. “Although an envisioned global quantum network which allows for secure and reliable transport of quantum information still demands a lot of research, the long-lived storage of quantum bits is one of the key technologies and we believe that the current improvements will bring us a significant step closer to its realization.” Olivia Meyer-Streng
 
© 2003-2018, Max-Planck-Gesellschaft, München



Prof. Ignacio Cirac receives Max-Planck Medal from the German Physical Society

Prof Ignacio Cirac, Director at the Max Planck Institute of Quantum Optics and head of the Theory Division, receives this honour for his groundbreaking contributions to the field of quantum information and quantum optics. His research topics range from fundamental mathematical calculations in quantum information theory, modelling of quantum many-body systems to concepts for the implementation of quantum optical systems.

Prof. Cirac will receive the prize – a handwritten certificate on parchment with the wording of the eulogy, and a gold medal bearing a portrait of Max Planck –at an award ceremony on 7 March 2018 on the occasion of the annual meeting of the DPG in Erlangen.

A strong focus of Prof. Cirac’s research is the simulation of the behaviour of quantum many-body systems with ultracold atoms in optical lattices. For example, such systems can serve as models for solid states crystals. This work will lead to a better understanding of macroscopic phenomena such as magnetism and superconductivity, but it also aims to get a deeper understanding about the true nature of light and matter.

At the centre of Prof. Cirac’s research is also the development of a new information theory based on the laws of quantum mechanics. New ways of controlling the world of atoms, molecules, and photons are being explored in order to exploit their quantum mechanical properties for storing and communicating quantum information with ever higher efficiency and security. These investigations are leading to new concepts for future quantum computers. The Theory Division of Prof. Cirac has for example developed new concepts for logical elements such as quantum gates that have already been implemented by experimental physicists. Furthermore, the group develops new algorithms for quantum communication, designs new quantum networks making use of the special properties of quantum particles, and creates new theoretical tools to characterize and quantify e.g. entanglement of remote quantum systems.

Information on Ignacio Cirac:

Professor Ignacio Cirac was born in the City of Manresa in 1965. He studied theoretical physics at the Universidad Complutense de Madrid where he received his PhD in 1991. He began his career in physics as a “Professor Titular” at the Universidad de Castilla-La Mancha where he stayed till 1996. In 1996 he became Professor at the department of Theoretical Physics at the University of Innsbruck. Since 2001 he has been Director at the Max Planck Institute of Quantum Optics and head of the Theory Division.

Professor Ignacio Cirac is a world-expert in the field of quantum information and quantum computation. In 2005 he was awarded the “Quantum Electronics Prize” of the European Science Foundation. In May 2006 he was the youngest ever winner of the renowned Royal Spanish Prince of Asturias Prize, and in the same year he received the International Quantum Communication Award together with Professor Peter Zoller (University of Innsbruck). In 2009 he shared the “Frontiers of Knowledge Award in Basic Sciences” of the Spanish BBVA Foundation as well as the Benjamin Franklin Medal of the Franklin Institute in Philadelphia (USA) with Professor Peter Zoller. In January 2013 he received both the Israeli Wolf Prize and the Niels Bohr Medal. In 2014, Prof. Cirac received the Honorary Doctor from the University of Zaragoza. In 2015 he got the Hamburg Prize for Theoretical Physics. In January 2016 he was elected ordinary member of the Royal-Spanish Academy of Sciences, and in October 2016 he received the Honorary Doctorate from the Universidad Europea de Madrid. In May this year Prof. Cirac became elected member of the Nationale Akademie der Wissenschaften Leopoldina.

Olivia Meyer-Streng

 

View original press release on MPQ webpage.

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