Quantum radar is overhyped and useless

There are many quantum technologies that get gushing write-up in science magazines, but have delivered no practical benefits. These include quantum cryptography, quantum computation, quantum teleportation, etc.

Another is quantum radar.

AAAS Science magazine reports:

A mini–arms race is unfolding in the supposed field, initiated by press reports in 2016 that China had built a quantum radar—potentially threatening the ability of stealthy military aircraft to hide in plain sight from conventional radars. ...

But to really make the scheme work, physicists must also preserve the retained microwave pulse until the reflected pulse (or the background replacing it) returns. Then, both pulses can be measured together in a way that enables the quantum waves to interfere. So far, however, nobody has done that. Instead, they’ve measured the retained pulse immediately and the returning pulse later, which in the experiments wipes out any gain from the quantum correlations.

Even if experimenters can overcome the technical hurdles, quantum radar would still suffer from a fatal weakness, researchers say. The entangled pulses of microwaves provide an advantage only when the broadcast pulses are extremely faint. The extra quantum correlations fade from prominence if pulses contain significantly more than one photon—which is overwhelmingly the case in real radar. “If you crank up the power, you won’t see any difference between the quantum and the classical,” Barzanjeh says. And cranking up the power is a much easier way to improve the sensitivity.

Such considerations suggest quantum radar will never be deployed for long-range uses such as tracking airplanes, says Fabrice Boust, a physicist at France’s aerospace agency, ONERA, who specializes in radar.

A lot of people are convinced that long-range entanglement is a form of quantum magic that will drive 21st century innovation. It is not.