Universal dynamics exposed by interaction quenches

Abstract

A quantum system prepared in an initial state that is not an eigenstate of its Hamiltonian will exhibit out-of-equilibrium dynamics. These dynamics can span multiple timescales, from the transient response of local correlations to the long-time equilibration of the system. The possibilities seem vast, depending on the initial state, the nature of the interactions and the observables being measured. Reporting in Nature Physics, Yuchen Li and colleagues have observed that the out-of-equilibrium dynamics of local magnetization correlations measured using solid-state nuclear magnetic resonance (NMR) follow a universal form¹.

The free induction decay signal records the temporal correlations of the local magnetization following the quench. Previous studies had suggested that the signal should follow a universal form, with the correlations decaying either as a simple exponential (exp(–γt)) or as a damped oscillation (cos(ωt)exp(–γt)) (ref. ^3,4). Careful NMR experiments on a wide range of systems, including polycrystalline xenon lattices and calcium fluoride^5,6, confirmed such behaviour. Additionally, a second oscillatory decay mode was observed in calcium fluoride⁷. However, the connection between these universal decays and the parameters of the underlying Hamiltonian remained unclear, limiting the possibility of predicting these universal modes in more general settings.