View from the arXiv: Jan 24-28 2022

Welcome to ‘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 As these are all preprints which have not yet been peer-reviewed, remember to take any claims and conclusions with a grain of salt and be sure to cast a critical eye over the work if you’re interested in more details.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 24th

Motional decoherence in ultracold Rydberg atom quantum simulators of spin models, by Zewen Zhang, Ming Yuan, Bhuvanesh Sundar, and Kaden R. A. Hazzard: Trapped ions have emerged as a promising candidate for quantum simulation and possibly even near-future quantum computers, and can be used to probe synthetic magnetic materials in a highly controlled environment. However, as with most other quantum simulation platforms, decoherence and noise are a limiting factor which must be addressed before these systems can see any form of practical use. Motivated by experiments showing an unexplained source of decoherence, this work proposes a novel form of decoherence related to the motion of the atoms in the trap which is consistent with the experimental observations, potentially allowing experimenters to alleviate or even eliminate these effects in the future now that they know what the possible cause may be. From the theoretical side, this work uses the discrete truncated Wigner approximation, which is a technique I’ve been curious about for some time (as it’s one I feel like I’m seeing more and more often), so reading this has given me a bit of a push to dig into the method in more detail and learn a bit more about it.

Dual dynamic scaling in deconfined quantum criticality, by Yu-Rong Shu, Shuai Yin: A quantum critical point occurs at zero temperature and marks the point at which a given system undergoes a continuous transition between two different phases. (Fun fact, my PhD started out as the study of quantum criticality before I fell down the rabibt hole of disorder…!) A deconfined quantum critical point, if I understand rightly, occurs when a two-dimensional system undergoes a continuous transition between two phases with different symmetries. Naively, phases with different symmetries should not be connected by a continuous phase transition, but at a deconfined quantum critical point there is a new so-called emergent symmetry which appears in order to facilitate the transition between two otherwise incompatible phases. This work studies the non-equilibrium relaxation dynamics in imaginary time, and in particular how the relaxation displays different scaling behaviours across the phase diagram, using this to estimate the location of the critical point as well as to study the properties of the phases on either side of it. The authors also make a connection to real-time dynamics and suggest experimental ways to test their predictions.

Jan 25th

Ultrafast Many-Body Dynamics in an Ultracold Rydberg-Excited Atomic Mott Insulator, by V. Bharti, S. Sugawa, M. Mizoguchi, M. Kunimi, Y. Zhang, S. de Léséleuc, T. Tomita, T. Franz, M. Weidemüller, and K. Ohmori: Tremendous progress has been made in controlling and synthesising quantum systems, particularly in ultracold atomic gases and trapped ion platforms, however there is always room for further development. In this work, the authors propose the use of ultrafast laser pulses to circumvent an effect known as the Rydberg blockade, which effectively limits how strongly trapped ions can interact with one another. This in turn affects the timescales on which the ions react, and this can be a problem for experiments as the ions only stay trapped for a finite amount of time. By using ultrafast laser pulses rather than the more conventional continuous wave lasers, the authors show a way to avoid the Rydberg blockade, allowing atoms to interact more strongly and undergo non-trivial quantum dynamics on shorter timescales, giving a new window onto the formation and propagation of quantum correlations in strongly interacting quantum matter.

Disorder-induced dynamical Griffiths singularities after certain quantum quenches, by José A. Hoyos, R. F. P. Costa, and J. C. Xavier: Griffiths effects usually occur in disordered systems where anomalous ‘rare regions’ can have highly atypical properties, dramatically modifying the thermodynamic properties of the material. In this work, the authors consider a dynamical analogue of this, where excitations generated by a sudden change of parameters (a quench) can remain localized and their dynamics are essentially decoupled from the dynamics of the rest of the system. This can lead to certain observables becoming non-analytic functions of time following a quench, contrary to what one would normally expect in a ‘well-behaved’ quantum system. The idea of dynamical Griffiths effects is certainly an interesting one, and I’m curious to see future works build on this concept.

Jan 26th

Symmetry-protected transport through a lattice with a local particle loss, by A.-M. Visuri, T. Giamarchi, and C. Kollath: Dissipation, in the form of the loss of particles or energy to an external environment, can dramatically change the properties of a quantum system otherwise in equilibrium, and has been much studied in recent years. This work takes a slightly different approach, studying a system already out of equilibrium and asking what the effect of dissipation would be, here in the form of particle loss on the central site of a one-dimensional chain of non-interacting fermions connected to a different thermal reservoir at each end. Remarkably, the authors show that there exist configurations where the conductance of the system can be almost unaffected by the dissipation, and demonstrate that certain symmetries of the system lead to this ‘protected transport’. It will be very interesting to see what happens when many-body interactions are included in this scenario!

Jan 27th

Generating Symmetry-Protected Long-Range Entanglement in Many-Body Systems, by Shovan Dutta, Stefan Kuhr, and Nigel R. Cooper: Quantum entanglement is the de facto language of much of modern condensed matter physics, and the currency of quantum information. Generating and preserving entanglement in quantum systems is crucial for the development of many quantum technologies, and it is critical that we have a reliable, robust way to do this. The authors of this paper have come up with a scheme by which spatially separated qubits can be driven into a maximally entanglement state by the application of a precise series of pulses, which could be realised in ultracold atomic gases by a laser beam and/or microwave field. This potentially allows experiments to construct strongly entangled states in a relatively simple manner, opening up possibilities for this entanglement to be used as a resource in quantum computing/information experiments.

Electron-Phonon Decoupling in Two Dimensions, by George McArdle, and Igor V. Lerner: Many-body localization (MBL) is a fascinating effect that many people (myself included) have been working on for some years now. It concerns the interplay of disorder and interactions in quantum mechanical systems which are isolated from their environments, and can fail to thermalise - put simply, MBL prevents any form of transport in the system, so neither heat nor particles can move from one area to another. MBL has been observed in ultracold atomic gases and trapped ion platforms, but not yet umambiguously seen in solid state materials. One complication is that electrons in solid state materials are strongly coupled to phonons (lattice vibrations), which act as an effective ’environment’ and causes the electrons to thermalize. This work sets out an intriguing proposition by which electrons can be decoupled from the thermal bath of phonons in low-dimensional solid state materials, and suggests promising candidate materials for the observation of MBL in experiments.

Jan 28th

Machine learning Markovian quantum master equations of few-body observables in interacting spin chains, by Francesco Carnazza, Federico Carollo, Dominik Zietlow, Sabine Andergassen, Georg Martius, and Igor Lesanovsky: The application of machine learning to all areas of physics is a fast developing – and at times, mildly controversial – area of research, but it is leading to some fascinating insights and new techniques. This work studies the dynamics of a many-body quantum system, split into a subsystem of interest and the rest of the system which plays the role of a thermal bath. The dynamical evolution of the subsystem can be formulated in terms of a quanum master equation (a method for studying the dynamics of open quantum systems). Here the authors use machine learning to extract properties of this master equation (specifically, from the Lindblad generator) such that they can obtain information about the stationary state of the system. While I am personally not yet convinced that machine learning techniques are reliable or powerful enough to stand alongside other established methods for studying many-body quantum systems, I am extremely curious to what they will become capable of in the future, and I found this approach conceptually very interesting indeed.