Quantum Radar

The F-35 Joint Strike Fighter—with an estimated price tag of $1.5 trillion—is the most expensive military project of all time. Much of the aircraft's value comes from its stealth capabilities. Lockheed Martin, the F-35's maker, spent a significant chunk of money developing new materials, techniques, and design features so the jet would be able to slip through an enemy's radar undetected.

In mid-September 2016, however, researchers from China's Electronic Technology Group Corporation revealed the world's first long-range quantum radar. Each particle this experimental system fires had an entangled partner held back at the sensor. Thanks to the quirky rules of quantum entanglement, anything these emitted particles encountered—including the radar-numbing F-35—would create an immediate reaction in their entangled partners back at the sensor. Thus, quantum radar would decloak the world's most expensive stealth fighter.

The Chinese announcement sent shockwaves through the defense community. Research groups from around the world—including one at Lockheed Martin—had been experimenting with quantum radar for the better part of a decade. Nobody had made headway anywhere close to this. A study from just a year earlier set the maximum effective range of quantum radar under 7 miles. And yet, the Chinese team claimed their model worked up to 61 miles out.

Chinese prototype of quantum radar, displayed at the 2018 Zhuhai Airshow. Credit: Steve Trimble @TheDEWLine

Not everyone was moved by the Chinese experiment. "They didn't supply details that would support their claim, so I conclude they didn't demonstrate what they claimed," says Seth Lloyd, a mechanical engineer at MIT whose 2008 Science paper set the theoretical foundation for quantum radar. What's more, he says this technology has a long way to go before it's viable for defense purposes.

According to Lloyd, anyone hoping to extend quantum radar's range needs to be very clever about how they work around the laws of physics. A quick primer on the theories underlying quantum radar can help explain why.

Quantum radar relies on a property called entanglement. Whenever two subatomic particles encounter one another, there's a chance their quantum states will become intertwined. Whatever happens to one will cause a reaction in the other. "To put it in anthropomorphic terms, they know more about one another than is possible to know in a classical system," says Lloyd.

Take, for instance, the property of spin. When talking about particles, spin doesn't refer to actual rotation, but is an analogy for the particle's polarity. Let's say you have two entangled particles, A and B. Until they are measured, the state of their spin is uncertain (rather, it exists in all states at once, thanks to superposition). Once you measure particle A, this puts the spin in a definite state. Let's say particle A's state is equal to 1. Now, if you measure particle B, you'll find that it has a spin equal to –1. (By the way, observing these particles also annihilates them. Quantum mechanics aren't for the sentimental.)

Though interesting, this entanglement does not allow a particle to send a signal to its entangled partner, as many news articles suggest. "Spooky action at a distance is a bit misleading," says Lloyd, in reference to Einstein's famous dismissal of the theory of quantum entanglement. "What Einstein actually said sounds better in German, and it means something slightly different. What matters is, if you send out an entangled particle, whatever happens out there conveys no information to the particle back in the lab."

What entanglement provides is a means of verifying that the photons picked up by the receiver are the same ones they sent out. This is because the particles that return still bear some trace of their entanglement to those that were kept behind. "Basically, what quantum entanglement does is enhance the signal-to-noise ratio for a given amount of power," says Lloyd. So, a small fraction of these photons could allow the operator to detect objects even in pea soup-levels of noise. Which is good, because no radar detector is a perfect gatekeeper. They all pick up noise.

In Lloyd's original 2008 paper, he proposes entangling photons using a beam splitter. Half of the photons are emitted out into a target area, while the other half are sent directly to the beam detector. This is where entanglement becomes useful. And even though the photons lose their entanglement—called decoherence—by background radiation, a formerly entangled pair is still more closely correlated than most of the noise coming back to the receiver. This useful paradigm is known as quantum illumination.

From Illumination to Radar

Quantum illumination works better when the levels of background radiation are higher relative to the power of the signal. Lloyd's framework, however, used photons in the visible spectrum, and there isn't a lot of useful background radiation in the visible range.

Drop to a lower frequency, however, and your background radiation increases dramatically. This is precisely what a multinational group of researchers proposed in 2015. Using a special converter, they said they could entangle photons with microwaves. The photons—the so-called idler particles—are sent directly to a detector, while microwaves are emitted. If any of the microwaves find their way back to the receiver, a converter changes them into photons before shunting them into the detector. Once at the detector, these microwave–photons could be compared to their formerly entangled companions.

This paper was the same one that set the limit for quantum radar at under 7 miles. The longer photons idle in the detector—waiting for their companion microwaves to return—the more loss they undergo, and lossy photons don't make for useful comparisons.

In August, the same group published new research, this time based on experimental results. "In our 2015 theory paper, we suggested using a microwave-to-optical photon conversion, then performing the analysis," says Seyed Shabir Barzanjeh, a postdoctoral researcher at the Institute of Science and Technology Austria, and the study's lead author. "But, in our recent experimental paper, we don't use any photon conversion and directly implemented the microwave quantum illumination/radar." The preprint of their paper, published on Arxiv, generated modest press, much of which focused on the defense applications of quantum radar.

However, Barzanjeh thinks those applications are overhyped. "Using our system as it is, I cannot imagine how this can be used for long-range applications and military industries," he says. Its limited range isn't the only problem, either. "For now, we only can detect the presence or absence of an object," he says. "We cannot talk about the shape or distance of the object."

That's hardly the sort of talk from someone who expects their field to render existing stealth technology moot. "The people doing the best experiments right now are saying this is hard to do, and it will take a while to have prototypes out there," says Lloyd.

As for the Chinese prototype—which the Electronic Technology Group Corporation displayed at the 2018 Zhuhai Airshow—Lloyd isn't the only one who has doubts. As reported in the South China Morning Post, several Chinese researchers not associated with the project expressed concerns. For one, the radar's capabilities were not backed up by publicly available research explaining their methods in detail. One outside researcher (who wished to remain anonymous) specifically called out the way the quantum radar team measured their photons, saying it could just be a "mathematical illusion."

Which isn't to say quantum radar is a pipe dream for the defense community. Secretive defense projects have been known to surprise the scientific community in the past. However, in the near term, quantum radar could be incredibly useful for close-up applications, such as medicine, where low-power imaging allows doctors to see inside a person without harming their tissue. "It's not only easier to do, but also might be more beneficial to society," says Lloyd.

Nick Stockton is a freelance writer based in Pittsburgh, Pennsylvania. He contributes to WIRED and Popular Science.