Long distance quantum illumination and ranging using polarization entangled photon pairs in a lossy environment

Abstract

Using polarization entangled photon pairs, we demonstrate a robust scheme for quantum illumination and ranging in a lossy environment. Entangled photon pairs are generated in a Sagnac interferometer configuration, yielding high-visibility two-photon polarization entanglement with a measured CHSH parameter of $S =2.802\pm0.002$. One of the photons from the entangled pair is retained as idler and the other one is directed into either of the two paths, namely reference and probe, of which probe is sent toward a distant object through a lossy free-space channel, and the reflected photons are collected after round-trip free-space propagation over distances approaching $1$ km. Remarkably, strong correlations are observed with CHSH values $S >2.6$ even when only a few tens of probe photons are returned, confirming the robustness of polarization entanglement under long-distance free-space propagation. This work reports the robustness of encoding photons in different basis before it is sent towards the object and recovery of polarization entanglement even after a kilometer-scale scattering from the objects, establishing a practical foundation for scalable quantum-assisted object detection and ranging.

Figures (7)

• Figure 1: Polarization-entanglement based quantum illumination protocol: One photon of each polarization-entangled pair (the idler) is sent directly to the measurement unit, while the other photon (the signal) is probabilistically routed into either the probe or the reference paths via a non-polarizing beam splitter (BS). Probe photons are transmitted through a sending telescope towards an object located in a noisy environment. The reflected photons from the object, together with noise, are collected by a receiving telescope after polarization filtering using a polarizing beam splitter (PBS) and are sent to the measurement unit. Photons traveling along the reference path bypass the object and are sent directly to the measurement unit, where both probe-idler and reference-idler correlations are independently analyzed via coincidence detection and correlation (CHSH value) measurements.
• Figure 2: Experimental setup for the quantum illumination based object detection. (a) Source: A type-II PPKTP crystal kept within a Sagnac configuration is pumped using a 405 nm laser that generates polarization entangled photon pairs at 808.049 nm. (b) Telescopic unit: A sending telescope in the probe path transmits the signal photons towards a distant object and a receiving telescope collects the signal reflected back from the object. (c) Object: The object is placed in a noisy environment at a certain distance from the source. (d) Measurement unit: Correlation measurement between idler photons and signal photons from either the probe or the reference path is performed to evaluate the $S$-values.
• Figure 3: Experimentally obtained visibility of the polarization-entangled state. The measured visibilities are $99.5\%$ in H/V basis (blue, green), and $98.4\%$ in A/D basis (orange, red).
• Figure 4: Photon counts per second (log scale) as a function of object distance for an object with $\sim96\%$ reflectivity : The brown dashed curve shows the probe photon transmission rate toward the object, while the green solid curve represents the photon collection rate after reflection from the object. The dotted blue curve indicates the theoretically expected collected photon rate after accounting for all system losses. The red dash-dotted curve corresponds to probe-idler coincidence count rate per second as a function of object distance.
• Figure 5: Photon counts per second (log scale) as a function of object distance for a scattering object with unknown reflectivity: The brown dashed line shows the probe photon transmission rate toward the object, while the green curve represents the photon collection rate after reflection from the object. The red dash-dotted line corresponds to probe-idler coincidence count rate as a function of object distance.