Linear growth of the entanglement entropy and the Kolmogorov-Sinai rate

TL;DR

This paper establishes a quantum analogue of the classical Kolmogorov-Sinai entropy rate, demonstrating linear entanglement entropy growth in unstable bosonic systems and exploring implications for quantum field theory and chaotic systems.

Contribution

It proves a quantum version of the entropy growth rate for bosonic systems with unstable quadratic Hamiltonians, including time-dependent cases with Floquet instabilities.

Findings

Entanglement entropy grows linearly with a rate bounded by the classical Kolmogorov-Sinai entropy.

The growth rate depends on Lyapunov exponents and subsystem choice.

Applications include quantum quenches, cosmological inflation, and chaotic quantum systems.

Abstract

The rate of entropy production in a classical dynamical system is characterized by the Kolmogorov-Sinai entropy rate $h_{\mathrm{KS}}$ given by the sum of all positive Lyapunov exponents of the system. We prove a quantum version of this result valid for bosonic systems with unstable quadratic Hamiltonian. The derivation takes into account the case of time-dependent Hamiltonians with Floquet instabilities. We show that the entanglement entropy $S_A$ of a Gaussian state grows linearly for large times in unstable systems, with a rate $\Lambda_A \leq h_{KS}$ determined by the Lyapunov exponents and the choice of the subsystem $A$. We apply our results to the analysis of entanglement production in unstable quadratic potentials and due to periodic quantum quenches in many-body quantum systems. Our results are relevant for quantum field theory, for which we present three applications: a scalar field in a symmetry-breaking potential, parametric resonance during post-inflationary reheating and cosmological perturbations during inflation. Finally, we conjecture that the same rate $\Lambda_A$ appears in the entanglement growth of chaotic quantum systems prepared in a semiclassical state.