Quantifying the source of enhancement in experimental continuous variable quantum illumination

Abstract

A quantum illumination protocol exploits correlated light beams to enhance the probability of detection of a partially reflecting object lying in a very noisy background. Recently a simple photon-number-detection-based implementation of a quantum illumination-like scheme was provided in Phys. Rev. Lett. 101, 153603 (2013), where the enhancement was preserved despite the loss of nonclassicality. In the present paper, we investigate the source for quantum advantage in that realization. We introduce an effective two-mode description of the light sources and analyze the mutual information (MI) as a quantifier of total correlations in the effective two-mode picture. In the relevant regime of a highly thermalized background, we find that the improvement in the signal-to-noise ratio achieved by the entangled sources over the unentangled thermal ones amounts exactly to the ratio of the effective MIs of the corresponding sources. More precisely, both quantities tend to a common limit specified by the squared ratio of the respective cross correlations. A thorough analysis of the experimental data confirms this theoretical result

Overcoming classical measurement limits through entanglement in photon number: an introduction

“…Losses and noise background do not affect the quantum enhancement. The improvement with respect to the classical case is also related to the mutual information of the twin beams which exceeds the one of classically correlated beams [7]. This application does not require entanglement, however it relies on non-classical correlations in the sense of the non-classicality of the Glauber-Sudarshan P function as demonstrated from the fact that the Cauchy-Schwartz parameter is always ε > 1.…”

“…03011-p. 7 If the phase between the TWB and the coherent beams is chosen properly, and for an initial phase shift of the two interferometers close to zero (but not exactly zero), as for example φ 1 = φ 2 ∼ 0.001, the use of TWB state leads to the result that the contribution to the uncertainty coming from the photon number noise is U (0) TWB ∝ 1/λ (when μ λ 1), representing a great improvement in accuracy of the interferometric scheme.…”

“…Among these technologies, quantum metrology and quantum imaging are of particular interest as they can improve the sensitivity and the resolution of measurements, by exploiting nonclassical features such as entanglement and squeezing. Here, we present some recent developments in overcoming classical limits by exploiting Quantum Optical Correlations (QOC) in photon number [2][3][4][5][6][7][8][9][10][11]. We will briefly review two protocols exploiting photon number correlation in order to detect either the absorption or the reflection of objects with enhanced sensitivity.…”

Quantum illumination for enhanced detection of Rayleigh-fading targets

“…Quantum illumination (QI) [1][2][3][4][5][6][7][8][9] uses entanglement to outperform the optimum classical-illumination (CI) system for detecting the presence of a weakly-reflecting target that is embedded in a very noisy background, despite that environment's destroying the initial entanglement [10]. With optimum quantum reception, QI's error-probability exponent-set by the quantum Chernoff bound (QCB) [13]-is 6 dB higher [4] than that of the optimum CI system, i.e., a coherent-state transmitter and a homodyne receiver.…”

Quantum illumination in the presence of photon loss

“…[6], we demonstrate that the non-Gaussian entanglement state derived from the biside photon subtraction could further extend the regime in which quantum illumination outperforms its classic variances. More recently, a quantum illumination experiment, based on photon number detection [7], has been carried out and has shown that illumination with a quantum protocol has clear advantages with respect to classical illumination [8].…”