We have built and evaluated a prototype quantum radar, which we call a quantum two-mode squeezing radar (QTMS radar), in the laboratory. It operates solely at microwave frequencies; there is no downconversion from optical frequencies.Because the signal generation process relies on quantum mechanical principles, the system is considered to contain a quantumenhanced radar transmitter. This transmitter generates a pair of entangled microwave signals and transmits one of them through free space, where the signal is measured using a simple and rudimentary receiver.At the heart of the transmitter is a device called a Josephson parametric amplifier (JPA), which generates a pair of entangled signals called two-mode squeezed vacuum (TMSV) at 6.1445 GHz and 7.5376 GHz. These are then sent through a chain of amplifiers. The 7.5376 GHz beam passes through 0.5 m of free space; the 6.1445 GHz signal is measured directly after amplification. The two measurement results are correlated in order to distinguish signal from noise.We compare our QTMS radar to a classical radar setup using conventional components, which we call a two-mode noise radar (TMN radar), and find that there is a significant gain when both systems broadcast signals at −82 dBm. This is shown via a comparison of receiver operator characteristic (ROC) curves. In particular, we find that the quantum radar requires 8 times fewer integrated samples compared to its classical counterpart to achieve the same performance.

We propose a novel protocol for quantum illumination: a quantum-enhanced noise radar. A two-mode squeezed state, which exhibits continuous-variable entanglement between so-called signal and idler beams, is used as input to the radar system. Compared to existing proposals for quantum illumination, our protocol does not require joint measurement of the signal and idler beams. This greatly enhances the practicality of the system by, for instance, eliminating the need for a quantum memory to store the idler. We perform a proof-of-principle experiment in the microwave regime, directly comparing the performance of a two-mode squeezed source to an ideal classical noise source that saturates the classical bound for correlation. We find that, even in the presence of significant added noise and loss, the quantum source outperforms the classical source by as much as an order of magnitude.Quantum illumination has recently gained attention as a possible avenue to improve the sensitivity of radar and other target detection technologies. 1,2 The approach takes advantage of strong signal correlations that can be created in electromagnetic beams using quantum processes. These quantum correlations, a form of entanglement, can be stronger than anything allowed by classical physics giving a "quantum advantage" to the detection process. A number of proposals exist to use these correlations in a wide range of quantum sensing applications with the goal of making precision measurements beyond the standard quantum limit. 3,4 Most of these applications require that the entire sensor system be low-noise and have negligible loss in order to maintain entanglement. Notably, quantum illumination seems to be very robust to the presence of background noise and loss, suggesting that it may have broader practical applications.In this Letter, we present measurements demonstrating the potential of a novel quantum illumination protocol that implements a form of noise radar. Noise radar has been studied in the classical regime because of, among other reasons, the inherent difficulty in detecting the noisy probe beam against the ambient thermal background noise. 5,6 As discussed below, our protocol relaxes a challenging requirement of existing protocols, namely, joint measurement. This greatly increases the practicality of our scheme compared to others. In a proof-ofprinciple experiment, we use the protocol to demonstrate a quantum enhancement in the detected signal-to-noise ratio of an order of magnitude when comparing the performance of an entangled-photon source to an ideal classical noise source that saturates the classical bound for correlation.At the heart of quantum illumination (QI) is a nonlinear quantum process known as parametric downconversion (PDC). In PDC, a strong pump beam with a high frequency, f p , is incident on a nonlinear medium, resulting in the production of two lower frequency beams, commonly referred to as the signal and idler, such that the frequencies of the produced beams, f s and f i , satisfy the relation f p = f s + f i ...

We investigate the use of correlated photon pair sources for the improved quantum-level detection of a target in the presence of a noise background. Photon pairs are generated by spontaneous fourwave mixing, one photon from each pair (the herald) is measured locally while the other (the signal) is sent to illuminate the target. Following diffuse reflection from the target, the signal photons are detected by a receiver and non-classical timing correlations between the signal and herald are measured in the presence of a configurable background noise source. Quantum correlations from the photon pair source can be used to provide an enhanced signal-to-noise ratio when compared to a classical light source of the same intensity.

Quantum two-mode squeezing (QTMS) radars and noise radars detect targets by correlating the received signal with an internally stored recording. A covariance matrix can be calculated between the two which, in theory, is a function of a single correlation coefficient. This coefficient can be used to decide whether a target is present or absent. We can estimate the correlation coefficient by minimizing the Frobenius norm between the sample covariance matrix and the theoretically expected form of the matrix. Using simulated data, we show that the estimates follow a Rice distribution whose parameters are simple functions of the underlying, "true" correlation coefficient as well as the number of integrated samples. We obtain an explicit expression for the receiver operating characteristic curve that results when the correlation coefficient is used for target detection. This is an important first step toward performance prediction for QTMS radars.

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