%0 Journal Article
%J New Journal of Physics
%D 2013
%T Dynamical quantum correlations of Ising models on an arbitrary lattice and their resilience to decoherence
%A Michael Foss-Feig
%A Kaden R A Hazzard
%A John J Bollinger
%A Ana Maria Rey
%A Charles W Clark
%X Ising models, and the physical systems described by them, play a central role in generating entangled states for use in quantum metrology and quantum information. In particular, ultracold atomic gases, trapped ion systems, and Rydberg atoms realize long-ranged Ising models, which even in the absence of a transverse field can give rise to highly non-classical dynamics and long-range quantum correlations. In the first part of this paper, we present a detailed theoretical framework for studying the dynamics of such systems driven (at time t=0) into arbitrary unentangled non-equilibrium states, thus greatly extending and unifying the work of Ref. [1]. Specifically, we derive exact expressions for closed-time-path ordered correlation functions, and use these to study experimentally relevant observables, e.g. Bloch vector and spin-squeezing dynamics. In the second part, these correlation functions are then used to derive closed-form expressions for the dynamics of arbitrary spin-spin correlation functions in the presence of both T_1 (spontaneous spin relaxation/excitation) and T_2 (dephasing) type decoherence processes. Even though the decoherence is local, our solution reveals that the competition between Ising dynamics and T_1 decoherence gives rise to an emergent non-local dephasing effect, thereby drastically amplifying the degradation of quantum correlations. In addition to identifying the mechanism of this deleterious effect, our solution points toward a scheme to eliminate it via measurement-based coherent feedback.
%B New Journal of Physics
%V 15
%P 113008
%8 2013/11/07
%G eng
%U http://arxiv.org/abs/1306.0172v1
%N 11
%! New J. Phys.
%R 10.1088/1367-2630/15/11/113008
%0 Journal Article
%J Optics Express
%D 2004
%T Quantum key distribution with 1.25 Gbps clock synchronization
%A J. C. Bienfang
%A A. J. Gross
%A A. Mink
%A B. J. Hershman
%A A. Nakassis
%A X. Tang
%A R. Lu
%A D. H. Su
%A Charles W Clark
%A Carl J. Williams
%A E. W. Hagley
%A Jesse Wen
%X We have demonstrated the exchange of sifted quantum cryptographic key over a 730 meter free-space link at rates of up to 1.0 Mbps, two orders of magnitude faster than previously reported results. A classical channel at 1550 nm operates in parallel with a quantum channel at 845 nm. Clock recovery techniques on the classical channel at 1.25 Gbps enable quantum transmission at up to the clock rate. System performance is currently limited by the timing resolution of our silicon avalanche photodiode detectors. With improved detector resolution, our technique will yield another order of magnitude increase in performance, with existing technology.
%B Optics Express
%V 12
%P 2011
%8 2004/05/17
%G eng
%U http://arxiv.org/abs/quant-ph/0405097v1
%N 9
%! Opt. Express
%R 10.1364/OPEX.12.002011