Trapped ions are used to determine the speed of propagation of correlations and the causal region to which they are confined in quantum many-body systems with medium- and long-range spin interactions. The speed at which information propagates in quantum many-body systems determines the overall behaviour of these systems. If the interactions between the system components are short-ranged, the dynamics are well understood and relatively straightforward to compute. Less clear is what happens when long-range interactions are present. Now two groups have used the exquisite control afforded by trapped atomic ions to explore experimentally how the interaction range influences the time evolution of quantum many-body systems. The maximum speed with which information can propagate in a quantum many-body system directly affects how quickly disparate parts of the system can become correlated1,2,3,4 and how difficult the system will be to describe numerically5. For systems with only short-range interactions, Lieb and Robinson derived a constant-velocity bound that limits correlations to within a linear effective 'light cone'6. However, little is known about the propagation speed in systems with long-range interactions, because analytic solutions rarely exist and because the best long-range bound7 is too loose to accurately describe the relevant dynamical timescales for any known spin model. Here we apply a variable-range Ising spin chain Hamiltonian and a variable-range XY spin chain Hamiltonian to a far-from-equilibrium quantum many-body system and observe its time evolution. For several different interaction ranges, we determine the spatial and time-dependent correlations, extract the shape of the light cone and measure the velocity with which correlations propagate through the system. This work opens the possibility for studying a wide range of many-body dynamics in quantum systems that are otherwise intractable.

Non-local propagation of correlations in quantum systems with long-range interactions