The signal mode a ̂ S is used to interrogate the presence ( i = 1) or absence ( i = 0) of a room-temperature object with total roundtrip reflectivity η. A quantum source generates and emits stationary entangled microwave fields in two separate paths. ( A) Schematic representation of microwave QI. We then compare the SNR with other detection strategies for the same signal path, i.e., the same signal photon numbers at the JPC output, which is also our reference point for the theoretical modeling. This enables an implementation of the phase-conjugate receiver that fully exploits the correlations of the JPC output fields without analog photodetection. Our experimental implementation of QI relies on linear quadrature measurements and suitable postprocessing to compute all covariance matrix elements from the full measurement record, as shown in previous microwave quantum optics experiments with linear detectors ( 28– 30). The reflection from the target a ̂ R is also detected, and the two measurement results are postprocessed to calculate the SNR for discriminating the presence or absence of the object. The generated signal microwave mode, with annihilation operator a ̂ S, is amplified to facilitate its detection and sent to probe a room-temperature target, while the idler mode a ̂ I is measured as schematically shown in Fig. ![]() We use a Josephson parametric converter (JPC) ( 24, 25) inside a dilution refrigerator for entanglement generation ( 26, 27). In this work, we implement a digital version of the phase-conjugate receiver of ( 22), experimentally investigating proof-of-concept QI in the microwave regime ( 23). In the low photon flux regime, where QI shows the biggest advantage, it could be suitable for extending quantum sensing techniques to short-range radar ( 18) and noninvasive diagnostic scanner applications ( 19). Although entanglement is lost in the round trip from the target, the surviving signal-idler correlations, when appropriately measured, can be strong enough to beat the performance achievable by the most powerful classical detection strategy. In the Gaussian QI protocol ( 12), the light is prepared in a two-mode squeezed vacuum state ( 3) with the signal mode sent to probe the target, while the idler mode is kept at the receiver. This is accomplished by probing the target with less than one entangled photon per mode, in a stealthy noninvasive fashion, which is impossible to reproduce with classical means. In QI, the aim is to detect a low-reflectivity object in the presence of very bright thermal noise. ![]() This is exactly the case with quantum illumination (QI) ( 11– 17) for its remarkable robustness to background noise, which, at room temperature, amounts to ∼10 3 thermal quanta per mode at a few gigahertz. Our results highlight the opportunities and challenges in the way toward a first room-temperature application of microwave quantum circuits.ĭespite this general picture, there are applications of quantum sensing that are naturally embedded in the microwave regime. Starting from experimental data, we also simulate the case of perfect idler photon number detection, which results in a quantum advantage compared with the relative classical benchmark. We implement a digital phase-conjugate receiver based on linear quadrature measurements that outperforms a symmetric classical noise radar in the same conditions, despite the entanglement-breaking signal path. We generate entangled fields to illuminate a room-temperature object at a distance of 1 m in a free-space detection setup. Here, we experimentally investigate the concept of quantum illumination at microwave frequencies. Its advantage is particularly evident at low signal powers, a promising feature for applications such as noninvasive biomedical scanning or low-power short-range radar. Quantum illumination uses entangled signal-idler photon pairs to boost the detection efficiency of low-reflectivity objects in environments with bright thermal noise.
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