A quantum radar that outperforms classical radar by 20%

Quantum technologies, a broad category of devices that make use of the laws of quantum physics, have the potential to do some jobs substantially better than conventional devices. As a result, physicists and technologists from all over the world have been working diligently to obtain this long-desired "quantum advantage" over conventional computer techniques.

A quantum radar that potentially greatly outperform all currently available radars based on conventional techniques was recently created by a research team at Ecole Normale Supérieure de Lyon, CNRS. This novel radar, which was described in a publication that appeared in Nature Physics, simultaneously monitors an entangled probe and the idler microwave photon states that emerge when the probe reflects from the target objects and merges with thermal noise.

According to Benjamin Huard, one of the researchers who conducted the study, "We invented a superconducting circuit in 2020 that was able to generate entanglement, store and manipulate microwave quantum states, and count the number of photons in a microwave field, among other things." We therefore came to the conclusion that it had all the characteristics we required to take on one of the most difficult tasks in microwave quantum metrology: proving a quantum advantage in radar sensing.

Previous research has attempted to create quantum radars that function better than traditional radars. Prior to this discovery, microwave radiation had not yet been used to take advantage of this quantum advantage, which was subsequently attained via optical devices.

Therefore, Huard and his team are the first to have created a microwave-based quantum radar that outperforms any conventional radar technology that has been documented thus far. Their radar operates outside the realm of conventional physics theories by taking use of correlations imprinted between two microwave radiations.

"Our radar generates quantum entanglement between a microwave resonator and a signal that is emitted towards a target that is hidden by a lot of microwave noise, such as in the atmosphere," said Huard. "If the target is real, it will reflect very little signal and very much noise. The amount of photons produced depends on whether the target is present or not after our gadget integrates this portion of intriguing signal with the field stored in the resonator. Finally, these photons are probed by an integrated microwave photon counter.

Previous studies demonstrated that, under conditions of equivalent signal strength and target noise, quantum correlations can speed up radar detection by up to four times. In preliminary tests, the researchers' microwave quantum radar accelerated radar detection by 20% in comparison to conventional radars.

Despite the straightforward nature of the operating circumstances, Huard claimed that it was quite difficult to make this demonstration work. "We conducted the entire experiment at 10 mK, away from open air, with only one unknown: whether the target was there or not. The technique requires signals considerably weaker than microwave photons to notice a quantum advantage, and we saw how much the initial signal must be purely entangled with the resonator to gain anything helpful. This is what I find most intimidating for direct applications in quantum radars.

In a series of experiments, Huard and his colleagues determined the quantum advantage of their radar across a broad range of parameters. These experiments showed that the initial purity of the entangled state between the idler and probe in their device can be a limiting issue, which should be taken into account when applying their radar in practical situations.

Huard added, "What I find most interesting is that we can get a quantum advantage even in a noisy environment where entanglement cannot survive." It is a unique case where non-classical correlations can be used to gain an advantage without causing any more entanglement.

This research team's most recent study has a significant impact on continuing initiatives to enhance the functionality of quantum radar technology. The methodology behind its operation may one day serve as a model for the creation of microwave quantum radars with even larger quantum advantage.

Huard continued, "I think there are many more applications where these non-classical yet entanglement-free correlations play a role that are yet to be uncovered. Now that we know how to use quantum resources to accomplish microwave sensing, we would like to learn more about it, such as how it applies to axion or electron spin resonance studies.