Irreversibility and the arrow of time in a quenched quantum system

TL;DR

This paper experimentally investigates quantum irreversibility by measuring entropy production in a spin-1/2 system, demonstrating its equivalence to the Kullback-Leibler divergence between a process and its reverse, thus linking microscopic quantum behavior to the arrow of time.

Contribution

It provides the first experimental verification that entropy production in a quantum system equals the entropic distance between a process and its time-reverse.

Findings

Entropy production equals the Kullback-Leibler divergence.

Experimental demonstration of microscopic irreversibility.

Quantitative link between entropy and time-reversal in quantum systems.

Abstract

Irreversibility is one of the most intriguing concepts in physics. While microscopic physical laws are perfectly reversible, macroscopic average behavior has a preferred direction of time. According to the second law of thermodynamics, this arrow of time is associated with a positive mean entropy production. Using a nuclear magnetic resonance setup, we measure the nonequilibrium entropy produced in an isolated spin-1/2 system following fast quenches of an external magnetic field and experimentally demonstrate that it is equal to the entropic distance, expressed by the Kullback-Leibler divergence, between a microscopic process and its time-reverse. Our result addresses the concept of irreversibility from a microscopic quantum standpoint.