Pieter W. Claeys



Quantum Fluids in Isolation

04 September 2020

Catch my recent seminar on Joshuah Heath's Quantum Fluids in Isolation virtual seminar series, where I talked and gesticulated about recent results on Thermalization and scrambling in dual-unitary circuit models. It was a fun experience, and I was glad to be part of such an interesting seminar series — do check out the upcoming and previous talks on the website.

Quantum lattice models with time evolution governed by local unitary quantum circuits can serve as a minimal model for the study of general unitary dynamics governed by local interactions. Although such circuit dynamics exhibit many of the features expected of generic many-body dynamics, exact results generally require the presence of randomness in the circuit. After a short introduction to general unitary circuit models, we discuss the class of dual-unitary circuits characterized by an underlying space-time symmetry. We show how in these models exact results can be obtained and related for the thermalization of correlations and the scrambling of out-of-time-order correlators.

Integrability and dark states in central spin models

28 August 2020

Our paper on Integrability and dark states in an anisotropic central spin model is now published in Physical Review Research as a Rapid Communication! My first last-author paper, and joint work with Tamiro Villazon and Anushya Chandran.

A key image and blurb are also highlighted on the PRResearch homepage.

Quantum eigenstates from classical Gibbs distributions

27 July 2020

Some interesting work Anatoli Polkovnikov is now on arXiv and submitted to SciPost Physics! In our new work on Quantum eigenstates from classical Gibbs distributions we investigate what happens when we try to express classical mechanics in the quantum mechanical language of states and operators and how to get from Gibbs ensembles to the Schrödinger equation?

Classical and quantum mechanics tend to be expressed in different languages, either continuous phase-space variables (position and momentum) or operators/states. Uncertainty leads to phase-space probability distributions (CM) or density matrices (QM). We can always try to express a density matrix as a classical probability distribution, asking the probability that a quantum particle has a given position or momentum. This leads to what's known as a 'Wigner quasiprobability distribution'. This behaves a lot like a probability distribution — but it can be negative! Still, if we treat it as a regular probability distribution to calculate physical observables we generally get sensible answers. Here we do the reverse: starting from a classical probability distribution, we map this to something that looks a lot like a quantum density matrix, where we can express observables such as position and momentum as the usual operators. Interestingly, something similar happens as in Wigner's function: while we can calculate expectation values of observables in the familiar way, this object is not quite a density matrix because its eigenvalues (probabilities) can be negative! It's easy to see why such quasiprobabilities need to be negative: if they weren't, we would have a classical uncertainty principle. For classical distributions with sufficient uncertainty, the negative quasiprobabilities disappear and we end up with an exact density matrix. One class of distributions with built-in uncertainty is that of Gibbs distributions. Given the temperature of a system, we know the classical probability of finding a system with given position and momentum, where higher temperatures lead to more uncertainty. Writing out the eigenvalue equation for their classical eigenstates, a saddle-point approximation returns the stationary Schrödinger equation! The approximation is controlled by the temperature and really works surprisingly well at not-too-low temperatures, and we explore this in the paper through various numerical and analytical examples.

We discuss how the language of wave functions (state vectors) and associated non-commuting Hermitian operators naturally emerges from classical mechanics by applying the inverse Wigner-Weyl transform to the phase space probability distribution and observables. In this language, the Schrödinger equation follows from the Liouville equation, with ℏ now a free parameter. Classical stationary distributions can be represented as sums over stationary states with discrete (quantized) energies, where these states directly correspond to quantum eigenstates. We show that this correspondence is particularly pronounced for canonical Gibbs ensembles, where classical eigenstates satisfy an integral eigenvalue equation that reduces to the Schrödinger equation in a saddle-point approximation controlled by the inverse temperature. We illustrate this correspondence by showing that some paradigmatic examples such as tunneling, band structures, and quantum eigenstates in chaotic potentials can be reproduced to a surprising precision from a classical Gibbs ensemble, without any reference to quantum mechanics and with all parameters (including ℏ) on the order of unity. Interestingly, it is now classical mechanics which allows for apparent negative probabilities to occupy eigenstates, dual to the negative probabilities in Wigner's quasiprobability distribution. These negative probabilities are shown to disappear when allowing sufficient uncertainty in the classical distributions.