Finite Speed of Quantum Scrambling with Long Range Interactions

In a locally interacting many-body system, two isolated qubits, separated by a large distance r, become correlated and entangled with each other at a time t >= r/v. This finite speed v of quantum information scrambling limits quantum information processing, thermalization, and even equilibrium correlations. Yet most experimental systems contain long range power-law interactions-qubits separated by r have potential energy V(r) proportional to r(-alpha). Examples include the long range Coulomb interactions in plasma (alpha = 1) and dipolar interactions between spins (alpha = 3). In one spatial dimension, we prove that the speed of quantum scrambling remains finite for sufficiently large alpha. This result parametrically improves previous bounds, compares favorably with recent numerical simulations, and can be realized in quantum simulators with dipolar interactions. Our new mathematical methods lead to improved algorithms for classically simulating quantum systems, and improve bounds on environmental decoherence in experimental quantum information processors.