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Optics Express

“Quantum illumination using polarization-entangled photon pairs for enhanced object detection: comment”: reply

Kanad Sengupta, K. Muhammed Shafi, Soumya Asokan, C. M. Chandrashekar

Published Aug 13, 2025• 0 citations

Abstract

The Comment ( 10.1364/OE.548099 ) is related to the theoretical model in our paper, Opt. Express 32 , 40150 ( 2024 ) 10.1364/OE.531674 , used to describe the experimental result. The role of coincidence counting in our experiment is central; it serves as a crucial post-selection mechanism to isolate the most likely entangled photon pairs even when a significant fraction of photons are lost in one arm. To model the experimentally obtained results, in the paper two theoretical approaches were presented to model the photon loss, one to detect the presence of object (using the constant S value we get even when coincidence photon counts rate reduce) and the other to estimate the reflectivity of the object (normalized S value change with decrease in coincidence rate with reflectivity η ). From the Comment, we gather that the second approach we have used to model photon loss and post-select is incorrect. To make it clear and complete, we have extended the first approach of using the complete Kraus operator to describe the dynamics from which we can detect the object and also estimate the reflectivity of the object. It accurately captures the experimental construct and is consistent with the results obtained from the measurable quantities. The second approach, using Kraus like operator M , which is erroneous, will be redundant.

DOI: 10.1364/oe.566362

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The Comment ( 10.1364/OE.548099 ) is related to the theoretical model in our paper, Opt. Express 32 , 40150 ( 2024 ) 10.1364/OE.531674 , used to describe the experimental result. The role of coincidence counting in our experiment is central; it serves as a crucial post-selection mechanism to isolate the most likely entangled photon pairs even when a significant fraction of photons are lost in one arm. To model the experimentally obtained results, in the paper two theoretical approaches were presented to model the photon loss, one to detect the presence of object (using the constant S value we get even when coincidence photon counts rate reduce) and the other to estimate the reflectivity of the object (normalized S value change with decrease in coincidence rate with reflectivity η ). From the Comment, we gather that the second approach we have used to model photon loss and post-select is incorrect. To make it clear and complete, we have extended the first approach of using the complete Kraus operator to describe the dynamics from which we can detect the object and also estimate the reflectivity of the object. It accurately captures the experimental construct and is consistent with the results obtained from the measurable quantities. The second approach, using Kraus like operator M , which is erroneous, will be redundant.

The paper [Opt. Express32, 40150 (2024)10.1364/OE.531674] investigates quantum illumination using polarization-entangled photon pairs for object detection in noisy environments. In this comment, we identify errors in the mathematical model for photon loss, particularly in the treatment of quantum entanglement under lossy channels. We show that the claimed robustness of polarization entanglement to photon loss contradicts established principles in quantum information theory, rendering the conclusions on detecting low-reflectivity objects unsupported.

Entangled light sources for illuminating objects offer advantages over conventional illumination methods by enhancing the detection sensitivity of reflecting objects. The core of the quantum advantage lies in effectively exploiting quantum correlations to isolate noise and detect objects with low reflectivity. This work experimentally demonstrates the benefits of using polarization-entangled photon pairs for quantum illumination and shows that the quantum correlation measure, using CHSH value and normalized CHSH value, is robust against losses, noise, and depolarization. We report the detection of objects with reflectivity (η) as low as 0.05 and an object submerged in noise with a signal-to-noise ratio of 0.003 using quantum correlation and residual quantum correlation measures, surpassing previous results. Additionally, we demonstrate that the normalized CHSH value aids in estimating the reflectivity of the detected object. Furthermore, we analyze the robustness of the correlation measure under photon attenuation in atmospheric conditions to show the practical feasibility of real-time applications.

This study compares the quantum bit error rate (QBER) performance of BB84, SARG04, and the entanglement-based BBM92 protocol in non-turbulent underwater optical channels. Clear, coastal, and turbid water types are considered under different background-illumination and receiver configurations. For BB84 and SARG04, channel attenuation and background-induced detections determine the quantum gain and QBER. For BBM92, photon loss is included through the two arm detection efficiencies, whereas the polarization state conditioned on two-photon detection is described by local depolarizing Kraus channels. The BBM92 QBER is obtained from the correlations in both measurement bases and from an exact partition of recorded events into true and false coincidences. Stochastic numerical estimates provide consistency checks of the analytical implementation. The results show that water turbidity and optical background reduce the distance below the adopted QBER threshold. They also show that a low conditional BBM92 QBER may coexist with a very small coincidence probability; therefore, the QBER-threshold distance alone does not represent a complete operational or secret-key-rate bound.

Detecting object with low reflectivity embedded within a noisy background is a challenging task. Quantum correlations between pairs of quantum states of light, though are highly sensitive to background noise and losses, offer advantages over traditional illumination methods. Instead of using correlated photon pairs which are sensitive, we experimentally demonstrate the advantage of using heralded single-photons entangled in polarization and path degree of freedom for quantum illumination. In the study, the object of different reflectivity is placed along the path of the signal in a variable thermal background before taking the joint measurements and calculating the quantum correlations. We show the significant advantage of using non-interferometric measurements along the multiple paths for single photon to isolate the signal from the background noise and outperform in detecting and ranging the low reflectivity objects even when the signal-to-noise ratio is as low as 0.03. Decrease in visibility of polarization along the signal path also results in similar observations. This will have direct relevance to the development of single-photon based quantum LiDAR and quantum imaging.