Entanglement, a key feature of quantum mechanics, is recognized for its non-classical correlations which have been shown to provide significant noise resistance in single-photon rangefinding and communications. Drawing inspiration from the advantage given by energy-time entanglement, we developed an energy-time correlated source based on a classical laser that preserves the substantial noise reduction typical of quantum illumination while surpassing the quantum brightness limitation by over six orders of magnitude, making it highly suitable for practical remote sensing applications. A frequency-agile pseudo-random source is realized through fiber chromatic dispersion and pulse carving using an electro-optic intensity modulator. Operating at a faint transmission power of 48 μW, the distance between two buildings 154.8182 m apart can be measured with a precision better than 0.1 mm, under varying solar background levels and weather conditions with an integration time of only 100 ms. These trials verified the predicted noise reduction of this system, demonstrating advantages over quantum illumination-based rangefinding and highlighting its potential for practical remote sensing applications.

Abstract Recently in [Phys. Rev. Lett. 114, 080503 (2015)] an electro-opto-mechanical converter has been used to evaluate microwave-optical entanglement, applicable in the quantum illumination techniques at microwave frequencies. Choosing this frequency domain is due to the fact that, microwaves have generally a more suitable spectral region for target detection than light waves. In this paper we aim to investigate the effect of nonlinear Kerr and cross-Kerr media on the microwave-optical entanglement degree in an electro-opto-mechanical transducer. We evaluate some key measures of the system, i.e., entanglement metrics, logarithmic negativity, coherent information and quantum discord. Our analysis demonstrates that, adding Kerr and cross-Kerr terms to the Hamiltonian of the electro-opto-mechanical system increases the entanglement metric, logarithmic negativity and quantum discord by approximately 3 to 4 times and the coherent information increases slightly, whenever the system possesses strong optical-mechanical coupling, low optical dissipation or both conditions. Conversely, when the system possesses weak optical-mechanical coupling, high optical dissipation or both conditions, it leads to weakening/death of all above-considered criteria. The constructive effect of Kerr media on the above quantumness signs makes the system more beneficial for quantum illumination purposes, which is an important factor in the quantum information science and technologies.

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.

The Comment (10.1364/OE.548099) is related to the theoretical model in our paper, Opt. Express32, 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.

Quantum optics has led to important advancements in our ability to prepare and detect correlations between individual photons. Its principles are increasingly translated into nanoscale characterization tools, furthering methods in spectroscopy, microscopy and metrology. In this Review, we discuss the rapid progress in this field driven by advanced technologies of single-photon detectors and quantum-light sources, including time-resolved single-photon counting cameras, superconducting nanowire single-photon detectors and entangled photon sources of increasing brightness. We emphasize emerging applications in super-resolution microscopy, measurements below classical noise limits and photon-number-resolved spectroscopy-a powerful paradigm for the characterization of nanoscale electronic materials. We conclude by discussing key technological challenges and future opportunities in materials science and bionanophotonics alike.

Quantum illumination leverages entanglement to surpass classical target detection, even in high-noise environments. Remarkably, its quantum advantage persists despite entanglement degradation caused by environmental decoherence. A central challenge lies in designing optimal receivers to exploit this advantage, with the correlation-to-displacement (C→D) conversion module emerging as a promising candidate. However, practical implementation remains technically demanding, particularly in realizing a low-noise programmable mode selector. In this work, we propose a scheme for the C→D module that incorporates a cavity-enhanced quantum pulse gate for programmable mode processing, achieving performance close to the theoretical optimum. This integrated framework paves the way for the realization of practical quantum illumination systems.

Abstract Quantum illumination is an emerging quantum technology that can detect whether a weakly reflecting target is present or absent in highly noisy environments. For practical purposes, in this paper we analyze the performance limit of a quantum illumination scheme and that of a classical illumination scheme in the presence of photon loss. The performance advantage of quantum illumination over classical illumination is addressed in settings of symmetric and asymmetric hypothesis testings. We analyze the effects of photon loss and discuss the relevant physical mechanisms. In addition, we demonstrate that noiseless linear amplification can be used to enhance the performance advantage. Our results are a foray into confirmation of the validity of quantum illumination and show more realistic performance advantage for practical applications.

Relation between quantum illumination and quantum parameter estimation

Abstract Microwave quantum illumination with entangled pairs of microwave signal and optical idler modes can achieve sub‐optimal performance with joint measurement of the signal and idler modes. Here, a testbed for microwave quantum illumination is proposed with an optical memory simulated with a delay line in the idler mode. It provides the amount of input two‐mode squeezing necessary to compensate for the loss of optical memory while maintaining a quantum advantage over a coherent state. When the memory is lossy, the input two‐mode squeezing has to be higher through high cooperativity in the optical mode. Under the testbed, a single‐mode phase conjugate receiver is proposed consisting of a low‐reflectivity beam splitter, an electro‐optomechanical phase conjugator, and a photon number‐resolving detector. The performance of the newly proposed receiver approaches the sub‐optimal quantum advantage of 3 dB. Furthermore, the receiver achieves the quantum advantage even with an on‐off detection while being robust against the loss of the memory.

Abstract We present a quantum ranging protocol that overcomes photon-loss limitations using optimized partially frequency-entangled states. By establishing the fundamental relationship between the degree of entanglement, channel transmission efficiency, and measurement precision, we demonstrate superclassical timing resolution in both lossless and lossy regimes. Theoretical analysis and numerical simulations reveal that: under a lossless channel, the precision gain increases with the degree of entanglement, approaching the Heisenberg limit. Importantly, in lossy channels, the precision gain is significantly influenced by both the channel transmission efficiency and the degree of entanglement. For transmission efficiencies above 50%, the proposed method provides up to 1.5 times the precision gain of classical methods when entanglement parameters are optimized. Moreover, by optimizing intra-group and inter-group covariances in the multi-structured entangled state, we achieve substantial precision gains even at low transmission efficiencies (~30%), demonstrating its robustness against loss. This work resolves the critical trade-off between entanglement-enhanced precision and loss-induced information degradation. Future implementations could extend to satellite-based quantum positioning, remote sensing, quantum illumination, and other fields that require high-precision ranging in lossy environments. The protocol establishes a universal framework for loss-tolerant quantum metrology, advancing the practical deployment of quantum-enhanced sensing in real-world applications.

Quantum two-mode squeezed (QTMS) radar, inspired by quantum illumination but without joint measurements in the receiver, has shown promise in target detection. However, the current prototype of quantum radar, using full-microwave superconducting components, faces challenges in achieving long-range performance. In this study, we propose the integration of an array of Josephson parametric amplifiers (JPAs) in a dilution refrigerator to enhance the performance of QTMS quantum radar. Through the evaluation of signal-to-noise ratio (SNR) and receiver operating characteristic (ROC) metrics, we demonstrate the effectiveness of this enhancing strategy. Our results indicate that the presence of an array of two JPAs significantly improves both the SNR and the detection probability compared to a single JPA configuration, thereby boosting the overall performance of QTMS radar. The key factor of this improvement is the cross-correlation between JPAs, which has a notable impact on the analytical outcomes of the quantum radar. This research provides valuable insights to engineers who want to optimize the design of advanced quantum QTMS radar systems.

Quantum illumination (QI) is an entanglement-based protocol for improving LiDAR/radar detection of unresolved targets beyond what a classical LiDAR/radar of the same average transmitted energy can do. Originally proposed by Seth Lloyd as a discrete-variable quantum LiDAR, it was soon shown that his proposal offered no quantum advantage over its best classical competitor. Continuous-variable, specifically Gaussian-state, QI has been shown to offer a true quantum advantage, both in theory and in table-top experiments. Moreover, despite its considerable drawbacks, the microwave version of Gaussian-state QI continues to attract research attention. A recent QI study by Armanpreet Pannu, Amr Helmy, and Hesham El Gamal (PHE), however, has: (i) combined the entangled state from Lloyd’s QI with the channel models from Gaussian-state QI; (ii) proposed a new positive operator-valued measurement for that composite setup; and (iii) claimed that, unlike Gaussian-state QI, PHE QI achieves the Nair–Gu lower bound on QI target-detection error probability at all noise brightnesses. PHE’s analysis was asymptotic, i.e., it presumed infinite-dimensional entanglement. The current paper works out the finite-dimensional performance of PHE QI. It shows that there is a threshold value for the entangled-state dimensionality below which there is no quantum advantage, and above which the Nair–Gu bound is approached asymptotically. Moreover, with both systems operating with error-probability exponents 1 dB lower than the Nair–Gu bound, PHE QI requires enormously higher entangled-state dimensionality than does Gaussian-state QI to achieve useful error probabilities in both high-brightness (100 photons/mode) and moderate-brightness (1 photon/mode) noise. Furthermore, neither system has an appreciable quantum advantage in low-brightness (much less than 1 photon/mode) noise.

Quantum illumination is an entanglement-based target detection protocol that provides quantum advantages despite entanglement-breaking noise. However, the advantage of traditional quantum illumination protocols is limited to impractical scenarios with low transmitted power and simple target configurations. Here, we address these challenges by introducing a quantum illumination network that leverages a transmitter array and a single receiver antenna. Thanks to multiple transmitters, quantum advantage is achieved with a high total transmitted power. Furthermore, the network resolves complex target configurations involving multiple unknown transmissivity or phase parameters. Despite the interference of different returning signals at the single antenna and photon loss due to multiple-access channels, we develop two types of measurement designs: one based on parametric amplification and the other on correlation-to-displacement conversion. Finally, we generalize the parameter estimation scenario to a general hypothesis testing scenario, where the six-decibel quantum illumination advantage is achieved at a much greater total probing power.

Quantum sensing promises to revolutionize sensing applications by employing quantum states of light or matter as sensing probes. Photons are the clear choice as quantum probes for remote sensing because they can travel to and interact with a distant target. Existing schemes are mainly based on the quantum illumination framework, which requires quantum memory to store a single photon of an initially entangled pair until its twin reflects off a target and returns for final correlation measurements. Existing demonstrations are limited to tabletop experiments, and expanding the sensing range faces various roadblocks, including long-time quantum storage and photon loss and noise when transmitting quantum signals over long distances. We propose a novel quantum sensing framework that addresses these challenges using quantum frequency combs with path identity for remote sensing of signatures (“qCOMBPASS”). The combination of one key quantum phenomenon and two quantum resources—namely, quantum-induced coherence by path identity, quantum frequency combs, and two-mode squeezed light—allows for quantum remote sensing without requiring quantum memory. The proposed scheme is akin to a quantum radar based on entangled frequency-comb pairs that uses path identity to detect, range, or sense a remote target of interest by measuring pulses of one comb in the pair that never traveled to the target but that contains target information “teleported” by quantum-induced coherence by path identity from the other comb in the pair that traveled to the target but is not detected. We develop the basic qCOMBPASS theory, analyze the properties of the qCOMBPASS transceiver, and introduce the qCOMBPASS equation—a quantum analog of the well-known LIDAR equation in classical remote sensing. We also describe an experimental scheme to demonstrate the concept using two-mode squeezed quantum combs. qCOMBPASS can strongly impact various applications in remote quantum sensing, imaging, metrology, and communications. These applications include detection and ranging of low-reflectivity objects, measurement of small displacements of a remote target with precision beyond the standard quantum limit (SQL), standoff hyperspectral quantum imaging, discreet surveillance from space with low detection probability (detect without being detected), very-long-baseline interferometry, quantum Doppler sensing, quantum clock synchronization, and networks of distributed quantum sensors. Published by the American Physical Society 2024

Quantum illumination with high-dimensional Bell states

In covert target detection, Alice attempts to send optical or microwave probes to determine the presence or absence of a weakly reflecting target embedded in thermal background radiation within a target region, while striving to remain undetected by an adversary, Willie, who is co-located with the target and collects all light that does not return to Alice. We formulate this problem in a realistic setting and derive quantum-mechanical limits on Alice's error probability performance in entanglement-assisted target detection for any fixed level of her detectability by Willie. We demonstrate how Alice can approach this performance limit using two-mode squeezed vacuum probes in the regime of small to moderate background brightness, and how such protocols can outperform any conventional approach using Gaussian-distributed coherent states. In addition, we derive a universal performance bound for nonadversarial quantum illumination without requiring the passive-signature assumption.

Abstract The authors consider a quantum radar which operates on the quantum illumination principle. The authors’ attention is focused on its function of target detection in a noisy environment. The role of the optical parametric amplifier (OPA) in detection is first examined by the authors, and a dual‐OPA design for more flexible combination of optimised gains is proposed, resulting in a detector substantially improved in its performance from the normally used 1‐OPA design. Then, the use of the entanglement information in the covariance matrix (CM) between the returned signal and idler beams for detection is considered, and a technique to extract such information is proposed. By employing some statistical relationships between positive definite matrices, the authors come up with a new target detection method. Numerical experiments confirm the superior detection performance of the CM detectors compared to that of the OPA detectors.

Most current microwave quantum illumination techniques rely on hybrid quantum systems to detect the presence of targets. However, real-world radar tasks are considerably more intricate than this simplistic model. Accurately determining physical attributes such as object speed, position, and azimuth is also essential. In this study, we explore azimuth detection using a quantum illumination approach based on a cavity-optomagnonics system and analyze the accuracy of azimuth detection in this framework. Our results indicate that this approach significantly outperforms classical microwave radar in azimuth detection within the parameters of current existing experiments. Additionally, we investigate the impact of Kerr nonlinearity of the YIG sphere on azimuth detection accuracy, revealing a clear improvement with the incorporation of Kerr nonlinearity.

Quantum illumination using non-Gaussian states with conditional measurements

The task of sensing the presence of a target object using a weak light source is challenging when the object is embedded in a noisy environment. One possibility is to use quantum illumination to do this, as it can outperform classical illumination in determining the object presence and range. This advantage persists even when both classical and quantum illumination are restricted to identical suboptimal object-detection measurements based on nonsimultaneous, phase-insensitive coincidence counts. Motivated by realistic experimental protocols, we present a theoretical framework for analyzing coincident multishot data with simple detectors. This approach allows for the often-overlooked noncoincidence data to be included, as well as providing a calibration-free threshold for inferring an object’s presence and range, enabling a fair comparison between different detection regimes. Our results quantify the advantage of quantum over classical illumination when performing target discrimination in a noisy thermal environment, including estimating the number of shots required to detect a target with a given confidence level. Published by the American Physical Society 2024