The short weird life—and potential afterlife—of quantum radar

You might call Jeffrey Shapiro the reluctant godfather of quantum radar. Twelve years ago, the electrical engineer at the Massachusetts Institute of Technology (MIT) helped develop the key concept underlying this scheme to dramatically increase radar’s sensitivity. But even he doesn’t think the technology will work. “There’s just a lot of problems that make it hard for me to believe that this system is going to be of any use,” Shapiro says. So he is both bemused and dismayed by the attention other researchers and funding agencies continue to lavish on it.

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. “I started working on this because there were government people coming to me and saying, ‘There are reports of quantum radar coming out of China. Is this real?’” says Christopher Wilson, a physicist at the University of Waterloo in Canada. His group and others have demonstrated elements of a quantum radar scheme, but only in limited experiments that a nonquantum system can still match.

The quantum radar story began in 2008, when Seth Lloyd, a quantum engineer at MIT, unveiled his concept of quantum illumination. Lloyd argued that you could more easily detect an object against a bright background if, instead of merely reflecting light off it, you exploited a quantum connection between particles called entanglement. Every photon has a frequency that determines its energy. Quantum theory says, weirdly, that a photon can have multiple frequencies at once—until it’s measured and “collapses” randomly to one frequency or another. Even weirder, two such photons can then be entangled so that their frequencies, although uncertain, are correlated: They are sure to be identical whenever they’re measured.

Lloyd calculated that an observer could more easily pick out an object by generating entangled pairs, shining one photon toward the object, keeping the other, and then measuring the retained and returning photons together in a particular way. Essentially, the entanglement correlations would make it harder to mistake a background photon for one reflected off a target. The signal to noise ratio would scale with the amount of entanglement: The more frequencies spanned by each photon in an entangled pair, the stronger the signal.

Lloyd’s calculation relied on a highly idealized form of entanglement. So that same year, he, Shapiro, and colleagues redid it for the real entangled light pulses that experimenters can generate with a special crystal that converts a single higher frequency pulse to two entangled pulses at lower frequencies. The pulses have no definite number of photons—just an average number—and they are “noisy,” like radio static. But thanks to the entanglement, the noise in the two pulses is highly correlated.

The researchers compared the sensitivity of a detector relying on the entangled pulses with a conventional one sending out single pulses of laser light, also known as coherent states. They found that the quantum effects boosted the signal-to-noise ratio by just a factor of four, less than they hoped for. “We were slightly disappointed,” says Si-Hui Tan, now a quantum information theorist at Horizon Quantum Computing. “Coherent states, they’re just so damned good!” she says.

Still, the calculation gave experimenters a target. In 2015, researchers at MIT demonstrated quantum illumination at optical frequencies, realizing a 20% increase in signal to noise. But that experiment had a major limitation. The whole idea was to detect an object against a bright background, but there’s very little optical background at room temperature—your surroundings don’t glow visibly. So the MIT team had to generate artificial background light.

Things are different in the microwave band, where radar works, says Johannes Fink, an experimental physicist at the Institute of Science and Technology Austria. At room temperature, microwaves stream from everything, even the air. “People are interested in the microwave because the background is always present,” he says. Stealth technologies hide military planes by suppressing their reflectivity at microwave frequencies so that the glow of the surroundings masks the plane’s reflections.

Quantum illumination seemed to promise a way to defeat stealth technologies. However, demonstrating the scheme with microwaves has proved daunting. Physicists can generate pairs of entangled microwave pulses from single ones using, instead of a crystal, a gizmo called a Josephson parametric converter. But that device only works at temperatures near absolute zero, which requires working within cryostats cooled with liquid helium.

Still, in 2019 Wilson and colleagues demonstrated that they could generate entangled microwaves and use them to detect an object within the same cryostat, as they reported in March 2019 in Applied Physics Letters. Fink; Shabir Barzanjeh, a physicist now at the University of Calgary; and colleagues performed a similar experiment, but amplified the signal pulse and ferried it out of the cryostat to detect a room temperature object, as they reported on 8 May in Science Advances.

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. And whatever system China may have developed, it almost certainly isn’t a quantum radar as commonly conceived, he says. “I am convinced that when they announced their quantum radar it was not working,” Boust says. “But they knew they would get a reaction.”

Fink says his personal goal remains scientific: demonstrating in the laboratory the true advantage—however small it may be—of entanglement for detecting objects hidden by glare. But the dream of fielding a quantum radar to detect stealth aircraft will likely fade away, says Giacomo Sorelli, a theorist at the Sorbonne University. “Taking out the long-range application of the technology will surely take out a lot of the interest of funding agencies,” he says.

Shapiro is less sure. This week, he notes, researchers again discussed quantum radar in a special session of the online Radar Conference of the Institute of Electrical and Electronics Engineers.