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arXiv:1512.02023 · Optics Express

Quantum correlation of light scattered by disordered media

Ilya Starshynov, Jacopo Bertolotti, Janet Anders

Published Feb 24, 2016• 10 citations
arXivHTMLPDF

Abstract

We study theoretically how multiple scattering of light in a disordered medium can spontaneously generate quantum correlations. In particular we focus on the case where the input state is Gaussian and characterize the correlations between two arbitrary output modes. As there is not a single all-inclusive measure of correlation, we characterise the output correlations with three measures: intensity fluctuations, entanglement, and quantum discord. We find that, while a coherent input state can not produce quantum correlations, any other Gaussian input will produce them in one form or another. This includes input states that are usually regarded as more classical than coherent ones, such as thermal states, which will produce a non-zero quantum discord.

DOI
10.1364/oe.24.004662

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We study theoretically how multiple scattering of light in a disordered medium can spontaneously generate quantum correlations. In particular we focus on the case where the input state is Gaussian and characterize the correlations between two arbitrary output modes. As there is not a single all-inclusive measure of correlation, we characterise the output correlations with three measures: intensity fluctuations, entanglement, and quantum discord. We find that, while a coherent input state can not produce quantum correlations, any other Gaussian input will produce them in one form or another. This includes input states that are usually regarded as more classical than coherent ones, such as thermal states, which will produce a non-zero quantum discord.
Multiple scattering is a very common phenomenon since it occurs any time a wave meets a disordered medium. As almost any natural object has random structure in one form or another, the variety of the processes involving multiple scattering spans from electronic transport in solids to propagation of sound in a forest. In principle, multiple scattering is completely deterministic, and in the absence of absorption also reversible, which means that the information encoded into the incident wave can be perfectly recovered. However, in practice, due to its extreme complexity we often consider this process to be random, which leads to information loss. Within this approach correlations can be an important instrument of information recovery, because they directly quantify the amount of knowledge we get about the wave in a particular point from the measurement performed in a different point. In the first part of this thesis we study a novel type of mesoscopic correlations between the light intensities at the opposite sides of an opaque scattering slab. We study its dependence on the scattering medium properties and the incoming light beam parameters. In the last chapter of the first part we show how this correlation can be used to retrieve non-invasively the information about the shape of an object placed behind the scattering medium. In the second part we switch to the quantum aspects of the light propagation inside the scattering materials. We show that certain class of quantum correlations, quantum discord, can be present in the multimode output state of the scattered light even when the input light is in a thermal state, which is commonly considered classical. We propose a non-classicality measure based on the strength of this correlation, applying it to characterize the advantage due to the quantum measurement in discrimination of two coherent states in their mixture.
Abstract In this paper, we investigate the creation of quantum correlations such as entanglement, the Gaussian quantum discord and quantum steering in a quantum system under consideration via the linear beam splitter with squeezed thermal and a single-mode Gaussian states at the inputs. This quantum system generates bright entangled output two-mode light. Specifically, the quantum entanglement can be enhanced through increasing the purity of the Gaussian state and the squeezing parameter at the input states. The quantum correlations reveal similar characteristics for each case. That means when we increase the non-classicality, purity and squeezing parameter, the quantum correlations show enhanced profile. On the other hand, upon increasing the mean thermal photon number at the input, the quantum correlations indicate inverse relation.
In the framework of the theory of open systems based on completely positive quantum dynamical semigroups, we give a description of the quantum correlations of a system consisting of two non-interacting modes embedded in a thermal environment. We describe the evolution of the quantum entanglement in terms of the covariance matrix for Gaussian input states. For all values of the temperature of the thermal reservoir, an initial separable squeezed thermal state remains separable for all times. In the case of an entangled initial squeezed thermal state, entanglement suppression (entanglement sudden death) takes place, for non-zero temperatures of the environment. The Gaussian quantum discord, which is a measure of all quantum correlations in the state, including entanglement, decays asymptotically in time under the effect of the thermal bath.
Abstract Quantum discord, an appropriate measure of quantum correlation, is analyzed in a two-mode Gaussian state of the cavity field evolved by a system of quantum beat laser. In the laser system, two arbitrary single-mode Gaussian states of the cavity fields and an external classical field couple to the atomic lasing medium. We calculate the time-dependent expression for the quantum discord both analytically and numerically, by following the reduced density matrix equation of the resulting cavity field. In this framework, we investigate the generation and control of Gaussian quantum discord in the two-mode Gaussian state of the cavity field at the output in terms of the purity and non-classicality of the two input cavity modes, the Rabi frequency of the classical driving field, the relative phase of the coupling parameters, and the damping rates of the cavity modes. The behaviour of quantum discord appears oscillatory due to the quantum beats in the output cavity field developed by the classical driving field in the medium as time passes. Moreover, we find that quantum discord switches on and off while adjusting the relative phase of the coupling parameters. Analysis of the analytical results agrees well with our numerical simulations.
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.