Light-cone-like spreading of correlations in a quantum many-body system

Abstract

In relativistic quantum field theory, information propagation is bounded by the speed of light. No such limit exists in the non-relativistic case, although in real physical systems, short-range interactions may be expected to restrict the propagation of information to finite velocities. The question of how fast correlations can spread in quantum many-body systems has been long studied. The existence of a maximal velocity, known as the Lieb-Robinson bound, has been shown theoretically to exist in several interacting many-body systems (for example, spins on a lattice)--such systems can be regarded as exhibiting an effective light cone that bounds the propagation speed of correlations. The existence of such a 'speed of light' has profound implications for condensed matter physics and quantum information, but has not been observed experimentally. Here we report the time-resolved detection of propagating correlations in an interacting quantum many-body system. By quenching a one-dimensional quantum gas in an optical lattice, we reveal how quasiparticle pairs transport correlations with a finite velocity across the system, resulting in an effective light cone for the quantum dynamics. Our results open perspectives for understanding the relaxation of closed quantum systems far from equilibrium, and for engineering the efficient quantum channels necessary for fast quantum computations.

Lieb-Robinson Bounds and the Simulation of Time-Evolution of Local Observables in Lattice Systems

“…In Ref. [11], the ballistic propagation of excitations and information propagation constrained by Lieb-Robinson bounds has been experimentally explored in systems of cold atoms.…”

Stationary behaviour of observables after a quantum quench in the spin-1/2 Heisenberg XXZ chain

“…Nonequilibrium dynamics in closed quantum systems, and in particular quantum quenches, have attracted much experimental [1][2][3][4][5][6] and theoretical attention in recent years. There is a growing consensus that integrable models exhibit important differences in behaviour as compared to non-integrable ones 41 .…”

Dynamic correlations, fluctuation-dissipation relations, and effective temperatures after a quantum quench of the transverse field Ising chain

“…The lack of thermalization to a Gibbs ensemble becomes apparent within this approach. Motivated by recent experimental advances in the field of cold atoms, the theoretical study of the non-equilibrium dynamics of isolated interacting many-body quantum systems is currently receiving increasing attention [1][2][3][4]. Among the several questions that have been addressed, a central one concerns the way in which a macroscopically large isolated system evolving with unitary quantum dynamics from a generic initial state approaches equilibrium.…”