Quantum advantage in microwave quantum radar

While quantum entanglement can enhance the performance of several technologies such as computing, sensing and cryptography, its widespread use is hindered by its sensitivity to noise and losses. Interestingly, even when entanglement has been destroyed, some tasks still exhibit a quantum advantage $Q$, defined by a $Q$-time speedup, over any classical strategies. A prominent example is the quantum radar, which enhances the detection of the presence of a target in noisy surroundings. To beat all classical strategies, Lloyd proposed to use a probe initially entangled with an idler that can be recombined and measured with the reflected probe. Observing any quantum advantage requires exploiting the quantum correlations between the probe and the idler. It involves their joint measurement or at least adapting the idler detection to the outcome of the probe measurement. In addition to successful demonstrations of such quantum illumination protocols at optical frequencies, the proposal of a microwave radar, closer to conventional radars, gathered a lot of interest. However, previous microwave implementations have not demonstrated any quantum advantage as probe and idler were always measured independently. In this work, we implement a joint measurement using a superconducting circuit and demonstrate a quantum advantage $Q>1$ for microwave radar. Storing the idler mitigates the detrimental impact of microwave loss on the quantum advantage, and the purity of the initial entangled state emerges as the next limit. While the experiment is a proof-of-principle performed inside a dilution refrigerator, it exhibits some of the inherent difficulties in implementing quantum radars such as the limited range of parameters where a quantum advantage can be observed or the requirement for very low probe and idler temperatures.