MajuLab Seminar by Alexia Auffèves, Institut Néel, CNRS, Grenoble, France

Title: A short story of quantum and information thermodynamics
Date/Time: 18-Nov, 04:30PM
Venue: Online Via Zoom

Abstract: This seminar is a fast journey through the build-up of key thermodynamical concepts, i.e. work, heat and irreversibility -- and how they relate to information. Born at the time of industrial revolution to optimize the exploitation of thermal resources, these concepts have been adapted to small systems where thermal fluctuations are predominant. Extending the framework to quantum fluctuations is a great challenge of quantum thermodynamics, that opens exciting research lines e.g. measurement fueled engines or thermodynamics of driven-dissipative systems. On a more applied side, it provides the tools to optimize the energetic consumption of future quantum computers.
  
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CQT Colloquium by Ron Folman, Ben-Gurion University of the Negev

Title: Matter-wave interferometers on the atom chip
Date/Time: 28-Oct, 04:00PM
Venue: Online via Zoom

Abstract: Matter-wave interferometry provides an excellent tool for fundamental studies as well as technological applications. In our group, several interferometry experiments have been done with a BEC on an atom chip [1] examining different effects. For example, we studied fluctuations in the nearby environment by an interference of atoms trapped in a magnetic lattice very close (5μm) to a room temperature surface [2,3]. We realized a new interferometry scheme of self-interfering clocks and showed, in a proof-of-principle experiment, how this could probe the interplay of QM and GR [4]. We also described a rule for “clock complementarity”, which we deduce theoretically and verify experimentally [5]. In the clock interferometer, we have observed phase jumps due to the existence of a geometric phase [6]. Furthermore, we realized Stern-Gerlach interferometry [7-10] despite several theoretical works which have shown over the years that fundamental barriers exist.

I will give a brief description of the advantages of the atom chip, and will then describe several of the interferometric schemes, and their connection to issues such as environmentally and gravitationally (red-shift) induced decoherence, as well as loss of coherence due to interferometer imprecision (the humpty-dumpty effect). I will conclude with an outlook concerning ideas for possible tests of exotic physics such as quantum gravity [11], and mention several speculations which we hope to examine in the future.

[1] M. Keil et al., “Fifteen years of cold matter on the atom chip: Promise, realizations and prospects”, Journal of Modern Optics 63, 1840 (2016).
[2] S. Zhou et al., “Robust spatial coherence 5m from a room temperature atom-chip”, Phys. Rev. A 93, 063615 (2016). 
[3] Y. Japha et al., “Suppression and enhancement of decoherence in an atomic Josephson junction”, New J. Phys. 18, 055008 (2016).
[4] Y. Margalit et al., “A self-interfering clock as a ‘which path’ witness”, Science 349, 1205 (2015).
[5] Z. Zhou et al., “Clock complementarity in the context of general relativity”, Classical and Quantum Gravity 35, 185003 (2018).
[6] Zhifan Zhou, Yair Margalit, Samuel Moukouri, Yigal Meir, and Ron Folman “An experimental test of the geodesic rule proposition for the non-cyclic geometric phase”, Science Advances 6, eaay8345 (2020).
[7] S. Machluf et al., “Coherent Stern-Gerlach momentum splitting on an atom chip”, Nature Communications 4, 2424 (2013).
[8] Y. Margalit et al., “Analysis of a high-stability Stern-Gerlach spatial fringe interferometer”, New J. Phys. 21, 073040 (2019).
[9] O. Amit, Y. Margalit, O. Dobkowski, Z. Zhou, Y. Japha, M. Zimmermann, M. A. Efremov, F. A. Narducci, E. M. Rasel, W. P. Schleich, R. Folman. “T3 Stern-Gerlach matter-wave interferometer”, Phys. Rev. Lett. 123, 083601 (2019).
[10] Mark Keil, Shimon Machluf, Yair Margalit, Zhifan Zhou, Omer Amit, Or Dobkowski, Yonathan Japha, Samuel Moukouri, Daniel Rohrlich, Zina Binstock, Yaniv Bar-Haim, Menachem Givon, David Groswasser, Yigal Meir, Ron Folman, “Stern-Gerlach Interferometry with the Atom Chip”, Invited review paper, in a book in honor of Otto Stern (Springer), https://arxiv.org/abs/2009.08112 (2021).
[11] Yair Margalit, Or Dobkowski, Zhifan Zhou, Omer Amit, Yonathan Japha, Samuel Moukouri, Daniel Rohrlich, Anupam Mazumdar, Sougato Bose, Carsten Henkel, Ron Folman, “Realization of a complete Stern-Gerlach interferometer: Towards a test of quantum gravity”, Science advances 7, eabg2879 (2021).
 

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CQT Talk - Quantum Machine Learning Journal Club Talk by Martin Larocca, Universidad de Buenos Aires, Los Alamos National Laboratory

Title: Theory of overparametrization in quantum neural networks
Date/Time: 11-Nov, 08:00PM
Venue: Online Via Zoom

Abstract:

The prospect of achieving quantum advantage with Quantum Neural Networks (QNNs) is exciting. Understanding how QNN properties (e.g., the number of parameters M) affect the loss landscape is crucial to the design of scalable QNN architectures. Here, we rigorously analyze the overparametrization phenomenon in QNNs with periodic structure. We define overparametrization as the regime where the QNN has more than a critical number of parameters Mc that allows it to explore all relevant directions in state space. Our main results show that the dimension of the Lie algebra obtained from the generators of the QNN is an upper bound for Mc, and for the maximal rank that the quantum Fisher information and Hessian matrices can reach. Underparametrized QNNs have spurious local minima in the loss landscape that start disappearing when M> Mc. Thus, the overparametrization onset corresponds to a computational phase transition where the QNN trainability is greatly improved by a more favorable landscape. We then connect the notion of overparametrization to the QNN capacity, so that when a QNN is overparametrized, its capacity achieves its maximum possible value. We run numerical simulations for eigensolver, compilation, and autoencoding applications to showcase the overparametrization computational phase transition. We note that our results also apply to variational quantum algorithms and quantum optimal control.

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