Taking the measure of quantum dynamics

The past decade has seen growing interest in identifying universal aspects of non-equilibrium dynamics in quantum many-body systems. Originally, this was motivated by the success of cold-atom experiments in creating isolated systems evolving under simple local Hamiltonians. A key theoretical question is to explain how such isolated systems come to equilibrium under their intrinsic unitary dynamics where the system is always in a pure state. This is now understood as a process of ‘scrambling’: over time, unitary evolution encodes any local non-thermal information about the initial state of the system into the global structure of the quantum pure state. Such global information is inaccessible to local observables, since degrees of freedom in any subsystem are entangled with those in the remainder of the system.