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arXiv:1310.6796 · Journal of Physics B Atomic Molecular and Optical Physics

Experimental verification of quantum discord in continuous-variable states

S Hosseini, Saleh Rahimi-Keshari, Jing Yan Haw, Syed M. Assad, Helen M. Chrzanowski, Jiří Janoušek, Thomas Symul, Timothy C. Ralph, Ping Koy Lam

Published Jan 3, 2014• 14 citations
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Abstract

We introduce a simple and efficient technique to verify quantum discord in unknown Gaussian states and a certain class of non-Gaussian states. We show that any separation in the peaks of the marginal distributions of one subsystem conditioned on two different outcomes of homodyne measurements performed on the other subsystem indicates correlation between the corresponding quadratures, and hence nonzero discord. We also apply this method to non-Gaussian states that are prepared by overlapping a statistical mixture of coherent and vacuum states on a beam splitter. We experimentally demonstrate this technique by verifying nonzero quantum discord in a bipartite Gaussian and certain non-Gaussian states.

DOI
10.1088/0953-4075/47/2/025503

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We introduce a simple and efficient technique to verify quantum discord in unknown Gaussian states and a certain class of non-Gaussian states. We show that any separation in the peaks of the marginal distributions of one subsystem conditioned on two different outcomes of homodyne measurements performed on the other subsystem indicates correlation between the corresponding quadratures, and hence nonzero discord. We also apply this method to non-Gaussian states that are prepared by overlapping a statistical mixture of coherent and vacuum states on a beam splitter. We experimentally demonstrate this technique by verifying nonzero quantum discord in a bipartite Gaussian and certain non-Gaussian states.
We introduce a simple and efficient technique to verify quantum discord in unknown Gaussian states and certain class of non-Gaussian states. We show that any separation in the peaks of the marginal distributions of one subsystem conditioned on two different outcomes of homodyne measurements performed on the other subsystem indicates correlation between the corresponding quadratures and hence nonzero quantum discord. We also demonstrate that under certain measurement constraints, discord between bipartite systems can be consumed to encode information that can only be accessed by coherent quantum interaction.
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In recent years the paradigm based on entanglement as the unique measure of quantum correlations has been challenged by the rise of new correlation concepts, such as quantum discord, able to reveal quantum correlations that are present in separable states. It is in general difficult to compute quantum discord, because it involves a minimization over all possible local measurements in a bipartition. In the realm of continuous-variable (CV) systems, a Gaussian version of quantum discord has been put forward upon restricting to Gaussian measurements. It is natural to ask whether non-Gaussian measurements can lead to a stronger minimization than Gaussian ones. Here we focus on two relevant classes of two-mode Gaussian states: squeezed thermal states and mixed thermal states, and allow for a range of experimentally feasible non-Gaussian measurements, comparing the results with the case of Gaussian measurements. We provide evidence that Gaussian measurements are optimal for Gaussian states.
Quantum discord provides a promising measure for the quantum correlation beyond entanglement, and may be responsible for the quantum speedup in certain quantum computing tasks. Here, we propose a necessary and sufficient condition for nonzero quantum discord in continuous variable systems, which is simple and easy to perform in terms of a marker QQQ. In order to obtain this condition, we introduce the continuous-variable local orthogonal basis of the operator space generalized from the orthogonal basis sets in local operator space for discrete variables. Based on this, we derive an explicit expression of the marker QQQ for all the two-mode states, and present examples of all Gaussian states and several non-Gaussian states. Our result provides a candidate for quantum correlation measures, and can be easily measured by a designed quantum circuit.
We extend the quantum discord to continuous variable systems and evaluate Gaussian quantum discord C(ϱ) for bipartite Gaussian states. In particular, for squeezed-thermal states, we explicitly maximize the extractable information over Gaussian measurements: C(ϱ) is minimized by a generalized measurement rather than a projective one. Almost all squeezed-thermal states have nonzero Gaussian discord: They may be either separable or entangled if the discord is below the threshold C(ϱ) = 1, whereas they are all entangled above the threshold. We elucidate the general role of state parameters in determining the discord and discuss its evolution in noisy channels.
Gaussian quantum discord is a measure of quantum correlations in gaussian systems. Using gaussian discord, we quantify the quantum correlations of a bipartite entangled state and a separable two-mode mixture of coherent states. We experimentally analyze the effect of noise addition and dissipation on gaussian discord and show that the former noise degrades the discord, while the latter noise for some states leads to an increase of the discord. In particular, we experimentally demonstrate the near death of discord by noisy evolution and its revival through dissipation.