Nature 481, 484–487 (2012)

The non-local nature of quantum mechanics is now firmly established experimentally, and quite a number of these studies have explored ways to harness quantum non-locality for computational or communication tasks. But in certain quantum systems aspects of locality do matter, as Marc Cheneau and colleagues demonstrate. They have measured the speed with which correlations spread in a quantum many-body system, and show that the dynamics is bounded by a maximal velocity that plays a role similar to the speed of light.

That there can be a limit to the speed with which correlations propagate in quantum many-body systems had been known from theoretical work. But how such bounds manifest in practical settings is, in most cases, difficult to predict. Thus the appeal of the work by Cheneau et al., who have directly measured the evolution of correlations in an ensemble of cold atoms confined to an optical lattice. They observe that the correlations spread along an effective 'light cone', characterized by a finite maximum velocity. The exploration of such bounds to non-relativistic quantum dynamics is not only of fundamental interest; they possibly also affect, for example, the speed at which quantum computations can be performed.