the outcomes

Funded by the European Union

General Eigenstate Thermalization via Free Cumulants in Quantum Lattice Systems

The Eigenstate-Thermalization-Hypothesis (ETH) is our favourite way to understand quantum thermalization and dynamics at equilibrium. Only recently attention has been paid to so-called general ETH, which accounts for higher-order correlations among matrix elements, and that we rationalized theoretically using the language of Free Probability (see below!).  In this work, we perform the first numerical investigation of the general ETH in physical many-body systems with local interactions by testing the decomposition of higher-order correlators into free cumulants. We perform exact diagonalization on two classes of local non-integrable (chaotic) quantum many-body systems: spin chain Hamiltonians and Floquet brickwork unitary circuits. We show that the dynamics of four-time correlation functions are encoded in fourth-order free cumulants, as predicted by ETH. Their non-trivial frequency dependence encodes the physical properties of local many-body systems and distinguishes them from structureless, rotationally invariant ensembles of random matrices.

Check it out on Arxiv!


Quantum bounds to the generalized Lyapunov exponents

We discuss the generalized quantum Lyapunov exponents Lq, defined from the growth rate of the powers of the square commutator. They may be related to an appropriately defined thermodynamic limit of the spectrum of the commutator, which plays the role of a large deviation function, obtained from the exponents Lq via a Legendre transform. We show that such exponents obey a generalized bound to chaos due to the fluctuation-dissipation theorem, as already discussed in the literature. The bounds for larger q are actually stronger, placing a limit on the large deviations of chaotic properties. Our findings at infinite temperature are exemplified by a numerical study of the kicked top, a paradigmatic model of quantum chaos.

Check it out on Arxiv!



The Eigenstate Thermaliztion Hypothesis and Free Probability Theory

The best way we have to understand quantum statistical mechanics is the Eigenstate Thermalization Hypothesis (ETH), which is an ansatz on the structure of observables when written in the eigenbasis of the Hamiltonian.  At equilibrium, there are correlations between observables at different times, and ETH has been recently shown to encode all the relevant correlations of matrix elements. However, the structure of these correlations has been elusive.

 In this work, we uncover the close relationship between this perspective on ETH and Free Probability theory, as applied to a thermal ensemble or an energy shell. Free Probability is a branch of math which generalizes probability to non-commuting variables. Check out the "Free probability blog" here. This mathematical framework allows one to reduce in a straightforward way higher-order correlation functions to a decomposition given by minimal blocks, identified as free cumulants, for which we give an explicit formula. The present results uncover a direct connection between the Eigenstate Thermalization Hypothesis and the structure of Free Probability, widening considerably the latter’s scope and highlighting its relevance to quantum thermalization.

This paper was published in Physical Review Letters. Check it out here or on Arxiv!