View from the arXiv: Jan 10-14 2022

Welcome to the second post of a new series, titled ‘View from the arXiv’, where each week I’ll put together a short list of new preprints which have appeared on arxiv.org during the week which I’ve found interesting. I’ll focus on the categories ‘Disordered Systems and Neural Networks’, ‘Quantum Gases’ and ‘Strongly Correlated Electrons’, and in particular the first two as these are the main areas of the arXiv which I follow. This is an entirely subjective list of things which appeal to me, and of course there are far too many interesting papers to be able cover all of them so I just choose one or two from each day to highlight here. As these are all preprints which have not yet been peer-reviewed, remember to take any claims and conclusions with a grain of salt and be sure to cast a critical eye over the work if you’re interested in more details. (And let’s face it, this caveat should be applied to any published work too…!)

Jan 10th

Strongly interacting trapped one-dimensional quantum gases: an exact solution, by Anna Minguzzi and Patrizia Vignolo: Exact solutions to complex problems are rare in physics, particularly in strongly quantum mechanical systems, but sometimes miracles can occur. This review surveys experimental and theoretical results for one-dimensional quantum gases (bosons, fermions and mixtures) with a particular emphasis on problems for which exact solutions exist. Interestingly, this even includes systems at non-zero temperatures, which I hadn’t realised could be described in an exact manner. I can absolutely foresee this being an article I come back to refer to again and again in the future!

Jan 11th

Quantum wake dynamics in Heisenberg antiferromagnetic chains, by Allen Scheie, Pontus Laurell, Bella Lake, Steven E. Nagler, Matthew B. Stone, Jean-Sebastian Caux, and D. Alan Tennant: Studying the real-time, spatially-resolved dynamics of a quantum spin system is a difficult challenge, one that (to my knowledge) is usually only achieved in ultracold atomic gases and trapped ion systems. It’s reminiscent of something I’ve worked on myself in the past, where we theoretically simulated the dynamics following a spin flip and then Fourier transformed in order to obtain spectral functions (this is known as quench spectroscopy). This paper takes the reverse approach, using experimental neutron scattering data to construct a spectral function in frequency- and momentum-space, and then Fourier transforming to obtain the space-time dynamics of the material following a quench. This is a pretty remarkable achievement, and I’d be very interested to see future work using similar techniques further examining the structure of the light cone. There are also some very intriguing hints of period-doubling behaviour, a phenomenon most closely linked to discrete time crystals.

Steady-State Quantum Zeno Effect of Driven-Dissipative Bosons with Dynamical Mean-Field Theory, by Matteo Seclì, Massimo Capone, and Marco Schiró: the study of many-body quantum systems subject to drive and dissipation is extremely demanding, and one of the most succesful methods developed to investigate these intriguing systems in recent years is dynamical mean-field theory (DMFT). This work studies a driven-dissipative Bose-Hubbard model, and shows that in the limit of strong dissipation it maps onto a hard-core Bose-Hubbard model which exhibits the surprising and counter-intuitive quantum Zeno effect, a phenomenon by which the effects of dissipation paradoxically seem to decrease as the dissipation strength itself is increased. The authors show that their DMFT method is ideal for describing and explaining the quantum Zeno regime; this is an excellent read for anyone who’d like to learn more about non-equilibrium DMFT.

Jan 12th

Cavity engineering of Hubbard U via phonon polaritons, by Brieuc Le Dé, Christian J. Eckhardt, Dante M. Kennes, and Michael A. Sentef: Ultrafast optical control of solid state materials is an exciting frontier in the generation and manipulation of exotic quantum mechanical effects, and in particular, experiments have seen signs of short-lived superconducting states in materials driven with lasers. Understanding the origin of this effect remains an open challenge, particularly as the short lifetime of the superconductivity makes it difficult to investigate in detail. This work explores an alternative setup, namely a material coupled to an optical cavity which leads to the generation of phonon polaritons which couple to the electrons. By altering the properties of the cavity, the authors find that they are able to tune the interaction between electrons in the system, essentially opening the door to a level of control that to date has been restricted to ultracold atomic gases and to my knowledge has never been achieved in solid state materials before. A very exciting development, and while I’m no expert on the superconducting aspects of this work, I am intrigued by the prospect of future experiments making use of this high level of microscopic control over the electrons in a solid.

Jan 13th

Overcoming the entanglement barrier in quantum many-body dynamics via space-time duality, by Alessio Lerose, Michael Sonner, and Dmitry A. Abanin: Another work in a recent series of papers by the Abanin group in Geneva developing an exciting new method for the study of non-equilibrium dynamics of quantum systems using influence matrices (IMs). This is an extremely intriguing method that may have a very wide regime of validity, contrary to tensor network methods which work most efficiently for systems with low entanglement. The key idea is based around the Feynman-Vernon influence functional method which has seen a lot of use in the study of open quantum systems, and involves separating the system into a subsystem of interest and its complement, which is treated somewhat like an external thermal bath. This work focuses on understanding and circumventing the temporal entanglement barrier (the generation of highly entangled states at intermediate timescales), and is a further step along the road of establishing the influence matrix technique as a major modern theoretical tool. One to watch in the years to come!

Jan 14th

Sub-diffusive Thouless time scaling in the Anderson model on random regular graphs, by Luis Colmenarez, David J. Luitz, Ivan M. Khaymovich, Giuseppe De Tomasi: The study of how - and if - closed quantum systems thermalize remains a central issue in modern condensed matter theory, and there have been a lot of extremely interesting recent developments, particularly in the context of many-body localization. This work investigates a related model, an Anderson model on a random regular graph (a form of highly-connected lattice, far from the one-dimensional limit where MBL is usually studied) and investigates a quantity known as the Thouless time. Loosely speaking, this is the time taken before the system starts to behave in universal way according to the principles of random matrix theory (which would typically imply thermalization). The authors compute the Thouless time from the spectral form factor and the power spectrum of their model, and study how the this time scales with various parameters. They argue that the results they observe suggest the emergence of a sub-diffusive regime that may be closely linked with the many-body localization transition in one-dimensional models; it will be interesting to see if further work can make this connection more concrete, as this work looks like a very nice step in the right direction.

The rise and fall, and slow rise again, of operator entanglement under dephasing, by David Wellnitz, Guillermo Preisser, Vincenzo Alba, Jerome Dubail, Johannes Schachenmaye: In modern physics, we often speak in the language of entanglement as it provides a powerful way to capture strongly quantum mechanical effects in a variety of systems. Starting from a state with zero entanglement, letting the state evolve in time and watching how the entanglement grows – and falls – can be an illumating way to probe quantum systems. In this work, the others look at an entanglement measure known as the ‘operator space entanglement entropy’ and observe how it changes over time in a one-dimensional spin chain the presence of dissipation. Naively, one might expect that dissipation should destroy entanglement over long timescales, but the results presented here suggest that this need not always be the case, and in fact the entanglement can grow logarithmically at long times. The authors connect this to a $U(1)$ conservation law, and provide some arguments as to the origin of this effect. This is an interesting and surprising result, with implications for a large variety of other quantum systems.