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SPIE Quantum Technologies

Optimisation of simple quantum illumination

Thomas Brougham, John Jeffers

Published Jun 10, 2024• 0 citations
Publisher

Abstract

We investigate the optimization of direct measurement twin beam (signal-idler) quantum illumination. We focus on each of the three main components of such a system in turn: the detectors, the light beam parameters and the information processing protocol. Surprisingly, there can be an advantage to having a signal detector whose quantum efficiency is significantly less than perfect. This advantage does not vanish in the quantum lidar limits of low object reflectivities or signal strengths. We show that decreasing the pulse separation, while keeping the photon flux fixed to retain the same degree of covertness, can improve target detection performance dramatically. Finally, we show that post-selecting on the idler detector firing is sub-optimal for target detection.

DOI
10.1117/12.3023882

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We investigate the optimization of direct measurement twin beam (signal-idler) quantum illumination. We focus on each of the three main components of such a system in turn: the detectors, the light beam parameters and the information processing protocol. Surprisingly, there can be an advantage to having a signal detector whose quantum efficiency is significantly less than perfect. This advantage does not vanish in the quantum lidar limits of low object reflectivities or signal strengths. We show that decreasing the pulse separation, while keeping the photon flux fixed to retain the same degree of covertness, can improve target detection performance dramatically. Finally, we show that post-selecting on the idler detector firing is sub-optimal for target detection.
Quantum illumination uses quantum correlations to improve the detection of objects in the presence of background noise. A simple and common practical approach to quantum illumination is to use threshold detectors to measure the idler mode. We investigate the optimization of these protocols. Surprisingly, we find that there can be an advantage to having a signal detector whose quantum efficiency is significantly less than perfect, one that does not vanish for low object reflectivities or signal strengths. We also show that decreasing the separation between pulses, while keeping the rate of photons transmitted per second fixed, can improve the performance, under appropriate conditions. We further show that postselecting on the idler detector firing is less efficient at object detection than not postselecting. Published by the American Physical Society 2024
Use of non-classical light in a quantum illumination scheme provides an advantage over classical illumination when used for LIDAR with a simple and realistic detection scheme based on Geiger-mode single photon detectors. Here we provide an analysis that accounts for the additional information gained when detectors do not fire that is typically neglected and show an improvement in performance of quantum illumination. Moreover, we provide a theoretical framework quantifying performance of both quantum and classical illumination for simple target detection, showing parameters for which a quantum advantage exists. Knowledge of the regimes that demonstrate a quantum advantage will inform where possible practical quantum LIDAR utilising non-classical light could be realised.
Use of non-classical light in a quantum illumination scheme provides an advantage over classical illumination when used for LIDAR with a simple and realistic detection scheme based on Geiger-mode single photon detectors. Here we provide an analysis that accounts for the additional information gained when detectors do not fire that is typically neglected and show an improvement in performance of quantum illumination. Moreover, we provide a theoretical framework quantifying performance of both quantum and classical illumination for simple target detection, showing parameters for which a quantum advantage exists. Knowledge of the regimes that demonstrate a quantum advantage will inform where possible practical quantum LIDAR utilising non-classical light could be realised.
Quantum Lidar is a future technology that can mean different things to different people. To some it means detection and identification of classical-type lidar signals reflected from an illuminated target at or near the single photon level. This is facilitated by advances in semiconductor detector technology that allow highly efficient photocounting. To others it is more stringent, requiring exploitation of covertness associated with being essentially undetectable by the target in a thermal background, whilst allowing the lidar operator to filter her own signal from this noise. Again there is nothing specifically quantum about this. Any source that mimics of the energy rate per bandwidth seen by the illuminated object in the direction of the source can appear covert. In order to exploit quantum optics one can use inherent correlations, whether entangled or otherwise between light beams. Simple correlated optical beams can do this in two ways, perhaps by storing one beam as a reference and later performing correlated detection with the return signal in a phase sensitive manner. This is the “gold standard” for quantum lidar, but will be difficult outside of laboratory situations. In microscopy such phase control might be possible, or perhaps in the radio or microwave region for a quantum radar. Less challenging is a scenario in which an immediate local measurement of one beam conditions the beam sent to the target, allowing the return signal to be sifted from noise more easily. Here we will consider such a simple system, providing a range equation for a basic quantum lidar if it is to be operated covertly or openly. Open operation allows increased quantum signal brightness but also provides a quantum advantage, unspoofability – the lidar operator can in principle recognise their own return signal. We will also provide an analysis of quantum lidar detection based on quantum hypothesis testing and use this to perform Monte Carlo simulation of both target detections and false positive avoidance in a noisy background. The theory will be applied to experimental data.
Quantum illumination is one of the main paradigms for implementing quantum radar in the low-frequency spectrum. Here, we discuss how to ease the open-air application of the protocol. I first define an experimentally feasible receiver for an entangled signal-idler transmitter. This consists of measuring heterodyne the received signal and adaptively measuring homodyne the idler, reaching a maximal quantum advantage of 3 dB in the error probability exponent with respect to the optimal classical strategy. Our receiver requires only a single tunable JPA. To relax the bandwidth requirement at the transmitter level, we discuss a sequential protocol that uses patches of modes sequentially to probe the target region. We show that, in a practical scenario, the sequential protocol needs two orders of magnitude less bandwidth with respect to the non-sequential protocol, while keeping the same quantum advantage. We finally briefly discuss possible applications of quantum illumination for backscatter communication and covert communication.
Abstract To detect a stealth target, one may directly probe it with a single photon and analyze the reflected signals. The efficiency of such conventional detection scheme can potentially be enhanced by quantum illumination, where entanglement is exploited to break the classical limits. The question is what is the optimal signal state for achieving the detection limit? Here, we address this question in a general discrete model, and derive a complete set of analytic solutions. For one-shot detection, the parameter space can be classified into three distinct regions, in the form of a “phase diagram” for both conventional and quantum illumination. Interestingly, whenever the reflectivity of the target is less than some critical value, all received signals become useless, which is true even if entangled resources are employed. However, there does exist a region where quantum illumination can provide advantages over conventional illumination; there, the optimal signal state is an entangled state with an entanglement spectrum inversely proportional to the spectrum of the environmental noise state and is, surprisingly, independent of the occurrence probability and the reflectivity of the object. The entanglement of the ideal probe state increases with the entropy of the environment; it becomes more entangled as the temperature of the environment increases. Finally, we show that the performance advantage cannot be fully characterized by any measure of quantum correlation, unless the environment is a complete mixed state.