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.
