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Johannes Meyer wins Quantum Futur Award

After Paul Faehrmann winning the Quantum Futur Award 2020, and our team the QHack 2020, Johannes Meyer has won the prestigious Quantum Futur Award 2021 of the German ministry for Research and Education BMBF. Warmest congratulations, very well deserved.

Aug 26, 2021

Work on generic quantum phases in PRX Quantum

The study of generic properties of quantum states has led to an abundance of insightful results. A meaningful set of states that can be efficiently prepared in experiments are ground states of gapped local Hamiltonians, which are well approximated by matrix product states. In this work, we introduce a picture of generic states within the trivial phase of matter with respect to their non-equilibrium and entropic properties: We do so by rigorously exploring non-translation-invariant matrix product states drawn from a local i.i.d. Haar-measure. We arrive at these results by exploiting techniques for computing moments of random unitary matrices and by exploiting a mapping to partition functions of classical statistical models, a method that has lead to valuable insights on local random quantum circuits. Specifically, we prove that such disordered random matrix product states equilibrate exponentially well with overwhelming probability under the time evolution of Hamiltonians featuring a non-degenerate spectrum. Moreover, we prove two results about the entanglement Renyi entropy: The entropy with respect to sufficiently disconnected subsystems is generically extensive in the system-size, and for small connected systems the entropy is almost maximal for sufficiently large bond dimensions. This work is in press in PRX Quantum .

Aug 09, 2021

Work on quantum readout in optical lattice systems in PRL

Quantum simulations with ultra-cold atoms in optical lattices open up an exciting path towards understanding strongly interacting quantum systems. Atom gas microscopes are crucial for this as they offer single-site density resolution, unparalleled in other quantum many-body systems. However, currently a direct measurement of local coherent currents is out of reach. In this work, we show how to achieve that by measuring densities that are altered in response to quenches to non-interacting dynamics, e.g., after tilting the optical lattice. For this, we establish a data analysis method solving the closed set of equations relating tunnelling currents and atom number dynamics, allowing to reliably recover the full covariance matrix, including off-diagonal terms representing coherent currents. The signal processing builds upon semi-definite optimization, providing bona fide covariance matrices optimally matching the observed data. We demonstrate how the obtained information about non-commuting observables allows to lower bound entanglement at finite temperature which opens up the possibility to study quantum correlations in quantum simulations going beyond classical capabilities. This work is in press at the Physical Review Letters .

Jul 13, 2021

Proof of the Brown Susskind conjecture

Quantifying quantum states' complexity is a key problem in various subfields of science, from quantum computing to black-hole physics. In our work , we prove a prominent conjecture by Brown and Susskind about how random quantum circuits' complexity increases. Consider constructing a unitary from Haar-random two-qubit quantum gates. Implementing the unitary exactly requires a circuit of some minimal number of gates - the unitary's exact circuit complexity. We prove that this complexity grows linearly in the number of random gates, with unit probability, until saturating after exponentially many random gates. Our proof is surprisingly short, given the established difficulty of lower-bounding the exact circuit complexity. Our strategy combines differential topology and elementary algebraic geometry with an inductive construction of Clifford circuits. This paper is also the first in the group to hit over 100 scites on Scirate .

Jun 16, 2021

Work on quantum field machines in PRX Quantum

Recent years have enjoyed an overwhelming interest in quantum thermodynamics, a field of research aimed at understanding thermodynamic tasks performed in the quantum regime. Further progress, however, seems to be obstructed by the lack of experimental implementations of thermal machines in which quantum effects play a decisive role. In this work , we introduce a blueprint of quantum field machines, which - once experimentally realized - would fill this gap. We provide a detailed proposal how to realize a quantum machine in one-dimensional ultra-cold atomic gases using a set of modular operations giving rise to a piston that can be coupled sequentially to thermal baths with the innovation that a quantum field takes up the role of the working fluid. We study the operational primitives numerically in the Tomonaga-Luttinger liquid framework proposing how to model the compression of the system during strokes of a piston and the coupling to a bath giving rise to a valve controlling phononic heat flow. By composing the numerically modeled operational primitives we design complete quantum thermodynamic cycles that are shown to enable cooling and hence giving rise to a quantum field refrigerator. The active cooling achieved in this way can operate in regimes where existing cooling methods become ineffective. We describe the consequences of operating the machine at the quantum level and give an outlook of how this work serves as a road map to explore open questions in quantum information, quantum thermodynamics and the study of non-Markovian quantum dynamics. This work has been published in PRX Quantum and has enjoyed a substantial coverage in the scientific press .

Jun 16, 2021

Work on quantum circuit complexity in PRL

Notions of circuit complexity and cost play a key role in quantum computing and simulation where they capture the (weighted) minimal number of gates that is required to implement a unitary. Similar notions also become increasingly prominent in high energy physics in the study of holography. While notions of entanglement have in general little implications for the quantum circuit complexity and the cost of a unitary, in this work, we discuss a simple such relationship when both the entanglement of a state and the cost of a unitary take small values, building on ideas on how values of entangling power of quantum gates add up. This bound implies that if entanglement entropies grow linearly in time, so does the cost. The implications are two-fold: It provides insights into complexity growth for short times. In the context of quantum simulation, it allows to compare digital and analog quantum simulators. The main technical contribution is a continuous-variable small incremental entangling bound. This work is in press at the Physical Review Letters . It is the 80th group publication in the Physical Review Letters.

Jun 15, 2021

Postdocs 2021

In 2021, we welcome Yihui Quek (Stanford), Sumeet Khatri (Baton Rouge and Stanford), Giacomo Guarnieri (Trinity College), Nora Tischler (Griffith), Austin Lund (Queensland), Philipp Schmoll (Mainz), Joschka Roffe (Sheffield), Steven Thomson (École Polytechnique), Ellen Derbyshire (Edinburgh), Spyros Sotiriadis (Ljubljana) as postdocs, most already being here. Very warm welcome again.

Jun 14, 2021

Work on machine learning of quantum optical experiments in PRX

Artificial intelligence (AI) is a potentially disruptive tool for physics and science in general. One crucial question is how this technology can contribute at a conceptual level to help acquiring new scientific understanding. Scientists used AI techniques to rediscover previously known concepts. So far, however, no examples of that kind have been reported that are applied to open problems for getting new scientific concepts and ideas. In work in press in the Physical Review X , we present an algorithm that can provide new conceptual understanding, and we demonstrate its applications in the field of experimental quantum optics. To do so, we make four crucial contributions. (i) We introduce graph-based representation of quantum optical experiments that can be interpreted and used algorithmically. (ii) We develop an automated design approach for new quantum experiments, which is orders of magnitudes faster than the best previous algorithms at concrete design tasks for experimental configuration. (iii) We solve several crucial open questions in experimental quantum optics which involve practical blueprints of resource states in photonic quantum technology and quantum states and transformations that allow for new foundational quantum experiments. Finally, and most importantly, (iv) the interpretable representation and enormous speed-up allow us to produce solutions that a human scientist can interpret and gain new scientific concepts from outright. We anticipate that will become an essential tool in quantum optics for developing new experiments and photonic hardware. It can further be generalized to answer open questions and provide new concepts in a large numbers of other quantum physical questions beyond quantum optical experiments. is a demonstration of explainable AI (XAI) in physics that shows a way how AI algorithms can contribute to science on a conceptual level.

Jun 09, 2021

A new major research network in Berlin on near-term quantum computing

We have set up a new major research network on near-term quantum devices and quantum computing within the Berlin University Alliance of Freie Universität Berlin , Humboldt-Universität zu Berlin , Technische Universität Berlin und Charité - Universitätsmedizin Berlin . The premise of this network is particularly fun and exciting since people working on quantum information theory are coming together with experts in combinatorical optimization, other readings of applied mathematics, with machine learning, high energy physics and quantum chemistry, as well as with experimentalists in quantum optics. So ideas of quantum software meet those of quantum hardware implementation. It is the first project within the Berlin Excellence Initiative of the Berlin University Alliance. We sincerely hope it will be good for the research field and for the Berlin academic landscape. A press release can be found here .

May 06, 2021

Our team wins QHack 2021

The team "Notorious FUB", a team composed of the PhD students of our group Peter-Jan Derks , Paul K. Faehrmann , Elies Gil-Fuster , Thomas S. Hubregtsen , Johannes Jakob Meyer , and the friend of the family David Wierichs have won the prestigious QHack 2021—the quantum machine learning hackathon. https://medium.com/xanaduai/qhack-the-quantum-machine-learning-hackathon-7f2cd7348e2b Participants from more than 85 countries tested their skills against one another to claim the top spots on the leaderboard on a topic of near-term quantum computing. At stake was more than bragging rights: top teams could unlock a share of $100k in credits for quantum computing platforms, early access to unreleased services, and internships at a world-famous scientific lab. A press release can be found here . Congratulations to this amazing achievement!

May 01, 2021

Work on quantum simulations in Nature Physics

Work on cold atomic quantum simulators probing "Gaussification dynamics" is going to press in Nature Physics . Gaussian models provide an excellent effective description of a plethora of quantum many-body systems ranging from a large variety of condensed matter systems all the way to neutron stars. Gaussian states are common at equilibrium when the interactions are weak. Recently it was proposed that they can also emerge dynamically from a non-Gaussian initial state evolving under non-interacting dynamics. In this work, we present the first experimental observation of such a dynamical emergence of Gaussian correlations in a quantum many-body system. For this, we monitor the connected fourth-order correlations during non-equilibrium dynamics. These dynamics are triggered by abruptly switching off the effective interaction between the collective degrees of freedom that we observe, while leaving the interactions between the microscopic constituents unchanged. Starting from highly non-Gaussian correlations, we observe a Gaussian description becoming increasingly accurate over time. In our closed system with non-interacting effective degrees of freedom, we do not expect full thermalization. This memory of the initial state is confirmed by observing recurrences of non-Gaussian correlations. Our study points to a natural way for Gaussian models to emerge in a wide class of (microscopically interacting) quantum many-body systems.

Jan 18, 2021

Two QIP talks

Two group contributions to QIP 2021 , the largest conference on quantum information processing, one on random quantum circuits and one on quantum metrology, have been accepted as talks. Congratulations.

Jan 04, 2021

Work on many-body localization in Communications Physics

Understanding the conditions under which physical systems thermalize is one of the long-standing questions in many-body physics. While it has been observed that generic quantum systems do thermalize, rather little is known about the precise underlying mechanism. Furthermore, instances in which thermalization is hindered for many-body systems have been uncovered, now known as many-body localization, offering promising insights into the mechanisms that underlie thermalization. In this work, we derive upper and lower bounds on the size of a heat bath required to thermalize a many-body localized system, for a broad class of collision models. To obtain these bounds, we employ a recently developed tool from quantum information theory known as the convex split lemma. We apply our results to the disordered Heisenberg chain, which we study numerically, and we characterize the robustness of the MBL phase in this system for the family of thermalization processes considered, in terms of the required bath size. Part of the significance of this work, published in Communications Physics (Nature) , stems from transferring tools from resource-theoretic quantum thermodynamics to the study of interacting quantum many-body systems.

Jan 04, 2021

Work on quantum advantages in PRL

Demonstrating a quantum computational speedup is a crucial milestone for near-term quantum technology. Recently, quantum simulation architectures have been proposed that have the potential to show such a quantum advantage, based on commonly made assumptions. The key challenge in the theoretical analysis of this scheme - as of other comparable schemes such as boson sampling - is to lessen the assumptions and close the theoretical loopholes, replacing them by rigorous arguments. In this work, we prove two open conjectures for these architectures for Hamiltonian quantum simulators: Anticoncentration of the generated probability distributions and average-case hardness of exactly evaluating those probabilities. The latter is proven building upon recently developed techniques for random circuit sampling. For the former, we develop new techniques that exploit the insight that approximate 2-designs for the unitary group admit anticoncentration. We prove that the 2D translation-invariant, constant depth architectures of quantum simulation form approximate 2-designs in a specific sense, thus obtaining a significantly stronger result. Our work, freshly published in the Physical Review Letters , provides the strongest evidence to date that Hamiltonian quantum simulation architectures are classically intractable.

Dec 10, 2020

Dominik Hangleiter defends his PhD with summa cum laude

Dominik Hangleiter, who has done his MSc project, a PhD and a short postdoc with us, has defended his PhD thesis on "Sampling and the complexity of nature" in a fulminant defense. Congratulations!

Nov 24, 2020

Marek Gluza defends his PhD in a fulminant viva

Marek Gluza has presented the work for his PhD thesis on "Non-equilibrium dynamics in quantum simulators" in a fulminant and vibrant defense, going a long way from natural quantum systems in condensed matter to highly engineered cold atomic quantum simulators. Warm congratulations!

Oct 29, 2020

Work on quantum simulators in PNAS

Quantum simulators promise to offer new insights into strongly correlated matter beyond what is accessible by means of classical computers. We propose dynamical quantum simulators (DQSs) as a method to simulate dynamical structure factors (DSFs) for system sizes considerably larger than what classical simulations can compute and provide complexity-theoretic evidence that they cannot be classically efficiently computed. Based on state-of-the-art experimental setups, we show how results from DQSs can be directly compared to experiments exploring properties of quantum materials. At the same time, we explore long-ranged spin systems: In particular, we show that the DSFs in DQSs can exhibit the signatures of excitation confinement in long-ranged models for which a comprehensive understanding is lacking. This work has been published in the Proceedings of the National Academy of Sciences (PNAS) . A press release can be found here .

Oct 02, 2020

Complexity of the sign problem in Science Advances

New work on the complexity of the sign problem in Quantum Monte Carlo (QMC) has been published in the Science Advances . QMC methods are the gold standard for studying equilibrium properties of quantum many-body systems - their phase transitions, ground and thermal state properties. However, in many interesting situations QMC methods are faced with a sign problem, causing the severe limitation of an exponential increase in the sampling complexity and hence the run-time of the QMC algorithm. In this work, we develop a systematic, generally applicable, and practically feasible methodology for easing the sign problem by efficiently computable basis changes and use it to rigorously assess the sign problem. Our framework introduces measures of non-stoquasticity that - as we demonstrate analytically and numerically - at the same time provide a practically relevant and efficiently computable figure of merit for the severity of the sign problem. We show that those measures can practically be brought to a good use to ease the sign problem. To do so, we use geometric algorithms for optimization over the orthogonal group and ease the sign problem of frustrated Heisenberg ladders. Complementing this pragmatic mindset, we prove that easing the sign problem in terms of those measures is in general an NP-complete task for nearest-neighbour Hamiltonians and simple basis choices by a polynomial reduction to the MAXCUT-problem. Intriguingly, easing remains hard even in cases in which we can efficiently assert that no exact solution exists. A press release can be found here. 

Aug 02, 2020

Work on multi-partite entanglement transformations in PRL

The theory of the asymptotic manipulation of pure bipartite quantum systems can be considered completely understood: The rates at which bipartite entangled states can be asymptotically transformed into each other are fully determined by a single number each, the respective entanglement entropy. In the multi-partite setting, similar questions of the optimally achievable rates of transforming one pure state into another are notoriously open. This seems particularly unfortunate in the light of the revived interest in such questions due to the perspective of experimentally realizing multi-partite quantum networks.In fact,  the question of what rates are achievable in multi-partite entanglement transformations emerged in the early 2000 years as one of the key questions in quantum information theory. It has not found a comprehensive answer to date. In this work, we report substantial progress on this long-standing question by deriving surprisingly simple upper and lower bounds on the rates that can be achieved in asymptotic multi-partite entanglement transformations. These bounds are based on ideas of entanglement combing and state merging. We identify cases where the bounds coincide and hence provide the exact rates. As an example, we bound rates at which resource states for the cryptographic scheme of quantum secret sharing can be distilled from arbitrary pure tripartite quantum states, providing further scope for quantum internet applications beyond point-to-point.This work has been published in the Physical Review Letters .

Jul 26, 2020

Review on certification and benchmarking in Nature Reviews Physics

With the rapid development of quantum technologies a pressing need has emerged for a wide array of tools for the certification and characterization of quantum devices. Such tools are critical since the powerful applications of quantum information science will only be realised if stringent levels of precision of components can be reached and their functioning guaranteed. This review in press in Nature Physics Reviews provides a brief overview of the known characterization methods for certification, benchmarking, and tomographic reconstruction of quantum states and processes, and outlines their applications in quantum computing, simulation, and communication. A press release can be found here .

Jun 01, 2020