Quantum target detection using entangled photons

TL;DR

This paper analyzes the performance of various quantum states, including entangled photons, in detecting targets amid thermal noise, revealing regimes where entangled states outperform or underperform compared to classical states.

Contribution

It provides analytic error-probability bounds for different quantum states in target detection, highlighting the conditions where entangled photons offer advantages over classical states.

Findings

N00N states outperform photon number states at low signal levels

Entangled SPDC photon pairs outperform coherent states in low signal-to-noise scenarios

Coherent states surpass entangled states at high signal intensities

Abstract

We investigate performances of pure continuous variable states in discriminating thermal and identity channels by comparing their M-copy error probability bounds. This offers us a simplified mathematical analysis for quantum target detection with slightly modified features: the object -- if it is present -- perfectly reflects the signal beam irradiating it, while thermal noise photons are returned to the receiver in its absence. This model facilitates us to obtain analytic results on error-probability bounds i.e., the quantum Chernoff bound and the lower bound constructed from the Bhattacharya bound on M-copy discrimination error-probabilities of some important quantum states, like photon number states, N00N states, coherent states and the entangled photons obtained from spontaneous parametric down conversion (SPDC). Comparing the $M$-copy error-bounds, we identify that N00N states indeed offer enhanced sensitivity than the photon number state system, when average signal photon number is small compared to the thermal noise level. However, in the high signal-to-noise scenario, N00N states fail to be advantageous than the photon number states. Entangled SPDC photon pairs too outperform conventional coherent state system in the low signal-to-noise case. On the other hand, conventional coherent state system surpasses the performance sensitivity offered by entangled photon pair, when the signal intensity is much above that of thermal noise. We find an analogous performance regime in the lossy target detection (where the target is modeled as a weakly reflecting object) in a high signal-to-noise scenario.