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

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.

💡 Research Summary

The authors present a comprehensive experimental demonstration of quantum illumination (QI) using polarization‑entangled photon pairs over a kilometer‑scale free‑space channel. A Sagnac interferometer housing a 10 mm periodically poled KTP crystal is pumped by a 405 nm continuous‑wave laser, generating degenerate photon pairs at 808 nm with a high pair‑production rate (~45 kpair · s⁻¹ · mW⁻¹) and heralding efficiency (~38 %). The source exhibits excellent polarization visibility (99.5 % in the H/V basis and 98.4 % in the diagonal/anti‑diagonal basis) and a CHSH Bell parameter of S = 2.802 ± 0.002, confirming near‑ideal entanglement.

In the QI protocol, one photon of each pair (the idler) is sent directly to a measurement module, while the other (the signal) is split by a 50:50 beam splitter into a reference arm and a probe arm. The reference arm bypasses the target and provides a real‑time benchmark of the source’s entanglement. The probe arm is directed through a sending telescope toward a distant reflective object (≈ 500 m away, corresponding to a 1 km round‑trip). Reflected photons are collected by a receiving telescope, coupled into a multimode fiber, and routed back to the same measurement module. Polarization analysis is performed with half‑wave plates and polarizing beam splitters in both arms, enabling simultaneous CHSH measurements for the reference‑idler and probe‑idler subsystems.

Theoretical modeling treats atmospheric loss as an exponential attenuation factor η(L) = Nₚ R exp(−aL), where R is the target reflectivity, a is the attenuation coefficient, and Nₚ accounts for optical and collection efficiencies. Because the CHSH correlation parameter E(α,β) = −cos 2(α+β) is a ratio of coincidence counts, the attenuation cancels out in the ideal case, predicting loss‑independent CHSH values. In practice, however, when the returned photon number drops to a few tens, statistical fluctuations in the coincidence counts reduce the observed S value. The authors therefore use the reference‑idler S as a calibration baseline to distinguish genuine loss of entanglement from statistical noise.

Experimental results show that even with only 20–30 returned probe photons per measurement interval, the probe‑idler CHSH parameter remains above S = 2.6, well beyond the classical bound of 2. This robust preservation of entanglement after 1 km of free‑space propagation and scattering from a realistic target demonstrates the practicality of polarization‑based QI in lossy environments. Moreover, the time‑of‑flight difference between idler and probe detections provides ranging information, achieving simultaneous object detection and distance measurement.

The work establishes several key advances: (1) a high‑quality, high‑flux polarization‑entangled source suitable for field deployment; (2) a dual‑arm architecture that continuously monitors source quality while probing a target; (3) experimental verification that CHSH violations survive extreme loss, confirming theoretical predictions of quantum advantage in illumination; and (4) a proof‑of‑concept for quantum‑enhanced radar‑like systems that could offer up to 6 dB error‑probability reduction over optimal classical illumination.

Future directions suggested include exploiting hyper‑entanglement (combining polarization with path, time‑bin, or orbital angular momentum), improving collection optics and detector efficiencies to increase the returned photon budget, and integrating real‑time error‑correction or adaptive measurement strategies. Overall, the paper provides a solid experimental foundation for scalable quantum‑assisted sensing, remote detection, and ranging in realistic, noisy, and lossy environments.