%0 Journal Article
%J Nature
%D 2014
%T Non-local propagation of correlations in long-range interacting quantum systems
%A Philip Richerme
%A Zhe-Xuan Gong
%A Aaron Lee
%A Crystal Senko
%A Jacob Smith
%A Michael Foss-Feig
%A Spyridon Michalakis
%A Alexey V. Gorshkov
%A Christopher Monroe
%X 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 correlated and how difficult the system will be to describe numerically. 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. However, little is known about the propagation speed in systems with long-range interactions, since the best long-range bound is too loose to give the correct light-cone shape for any known spin model and since analytic solutions rarely exist. In this work, we experimentally determine the spatial and time-dependent correlations of a far-from-equilibrium quantum many-body system evolving under a long-range Ising- or XY-model Hamiltonian. For several different interaction ranges, we extract the shape of the light cone and measure the velocity with which correlations propagate through the system. In many cases we find increasing propagation velocities, which violate the Lieb-Robinson prediction, and in one instance cannot be explained by any existing theory. Our results demonstrate that even modestly-sized quantum simulators are well-poised for studying complicated many-body systems that are intractable to classical computation.
%B Nature
%V 511
%P 198 - 201
%8 2014/7/9
%G eng
%U http://arxiv.org/abs/1401.5088v1
%N 7508
%! Nature
%R 10.1038/nature13450
%0 Journal Article
%J Physical Review A
%D 2013
%T Experimental Performance of a Quantum Simulator: Optimizing Adiabatic Evolution and Identifying Many-Body Ground States
%A Philip Richerme
%A Crystal Senko
%A Jacob Smith
%A Aaron Lee
%A Simcha Korenblit
%A Christopher Monroe
%X We use local adiabatic evolution to experimentally create and determine the ground state spin ordering of a fully-connected Ising model with up to 14 spins. Local adiabatic evolution -- in which the system evolution rate is a function of the instantaneous energy gap -- is found to maximize the ground state probability compared with other adiabatic methods while only requiring knowledge of the lowest $\sim N$ of the $2^N$ Hamiltonian eigenvalues. We also demonstrate that the ground state ordering can be experimentally identified as the most probable of all possible spin configurations, even when the evolution is highly non-adiabatic.
%B Physical Review A
%V 88
%8 2013/7/31
%G eng
%U http://arxiv.org/abs/1305.2253v1
%N 1
%! Phys. Rev. A
%R 10.1103/PhysRevA.88.012334
%0 Journal Article
%J Physical Review Letters
%D 2013
%T Quantum Catalysis of Magnetic Phase Transitions in a Quantum Simulator
%A Philip Richerme
%A Crystal Senko
%A Simcha Korenblit
%A Jacob Smith
%A Aaron Lee
%A Rajibul Islam
%A Wesley C. Campbell
%A Christopher Monroe
%X We control quantum fluctuations to create the ground state magnetic phases of a classical Ising model with a tunable longitudinal magnetic field using a system of 6 to 10 atomic ion spins. Due to the long-range Ising interactions, the various ground state spin configurations are separated by multiple first-order phase transitions, which in our zero temperature system cannot be driven by thermal fluctuations. We instead use a transverse magnetic field as a quantum catalyst to observe the first steps of the complete fractal devil's staircase, which emerges in the thermodynamic limit and can be mapped to a large number of many-body and energy-optimization problems.
%B Physical Review Letters
%V 111
%8 2013/9/5
%G eng
%U http://arxiv.org/abs/1303.6983v2
%N 10
%! Phys. Rev. Lett.
%R 10.1103/PhysRevLett.111.100506