Stealth Bombers and Cancer Cells Be Warned: Quantum Radar Has Gone Microwave

​Like a lot of things with "quantum" in their name, quantum radar picks up where its classical analog ends. Stealth fighters? Cancer cells? Quantum radar can spot them with relative ease. But, again, like most things with quantum in their name, the technology is only now coming into reach.

Researchers at the University of York have developed a new system that promises to open up quantum radar technology in a new, practical way. As described by ​a paper in the current Physical Review Letters, their system depends on a new sort of electromagnetic frequency converter, one allowing the coupling (or entanglement) of beams in optical wavelengths with beams in the microwave spectrum.

The result is a radar system that can both generate entangled microwave-optical beams, as during signal emission, and convert the received microwave beams back into optical wavelengths.

Quantum radar is still fairly out-there idea. Lockheed Martin holds ​a generic patent in Europe on the theorized technology, but the defense contractor doesn't offer much in the way of possible implementations. Using "entangled quantum particles," Lockheed's would-be system, should allow its users to "visualise useful target details through background and/or camouflaging clutter, through plasma shrouds around hypersonic air vehicles, through the layers of concealment hiding underground facilities, [and find] IEDs [improvised explosive devices], mines and other threats--all while operating from an airborne platform." That's not a particularly modest declaration.

The gist of the idea (also known as "quantum illumination") is that, through quantum entanglement, beams of extremely high-frequencies, like microwaves, might be used to image cloaked objects very far away. Currently, this is an impossibility as beams like this don't travel so well across big distances.

That is, a microwave radar beam might do well enough when it's reflecting against some well-defined, conventional surface, where the returning beam is sufficiently strong to persist through atmospheric thermal background noise. But a stealth surface, where this reflection is minimized, reflects a radar beam only very weakly. A detection is made, but the returning signal isn't powerful enough to make the trip back. That's the whole idea of stealth technology: not invisibility, but good-enough dampening.

The concept behind quantum radar is quantum entanglement. This is where one or more particles or particle collections are put into the same quantum state, such that they're "sharing" the state or even becoming indistinguishable from each other, with the effect being that a distant particle can influence (in a sense) its nearby entangled partner. It gives the illusion (at least) of a sort of superliminal communication.

With quantum radar, microwave beams are entangled with optical (laser) beams, such that the outgoing "probe" beam is effectively tagged in a way that when it completes its round-trip to the target and back, the returning probe beam can be discerned from the background noise by its correlation with the optical beam, which stayed put. "A light beam is split into two beams that are quantum-mechanically entangled, meaning that the quantum states of the photons in each beam are strongly correlated," Phillip Ball writes in ​an accompanying American Physical Society perspective.

"In quantum illumination," Ball explains, "one of the two beams goes straight to a detector, but the other (the probe) is sent out to sense a target. If the target is present, an echo is reflected from it and travels back to the detector, where it interferes with the idler beam. Even if the entanglement between the two beams is broken up by noise in the environment, some residual correlations remain that affect the interference, so that probe photons can be distinguished from background photons. This makes it possible to identify reflections from the target even when this echo is very weak." So, it's a quantum-enabled method of signal amplification.

Even the general idea of quantum radar/illumination is still very new and was demonstrated experimentally for the first time in 2013, using only optical wavelengths of light. The new work achieves the coupling of microwave wavelengths and optical using a pair of cavities storing either variety of radiation (microwave and optical), which are linked together by a nanoscale vibrating element. Once entangled via the element, the microwave probe beam can be sent out, while its optical partner/sibling waits ready to "verify" the returning radiation.

"Thanks to this converter, the target region can be interrogated at a microwave frequency, while the quantum-illumination joint measurement needed for target detection is made at optical frequency, where the high-performance photodetectors needed to obtain QI's performance advantage are available," the York researchers offer in the current paper.

The system has the additional advantage of being interference-proof, in theory, as any disturbances (measurements) made on or to the probe beam would have the effect of knocking it out of coherence with the optical beam. This is a lot like the security advantage offered by quantum encryption. If the beam was "hacked," it would show.

As for the future, the technology as applied to other wavelengths could mean much more precise medical imaging at much lower energies of electromagnetic radiation.

"Finally," the authors conclude, "extending our results to lower frequencies (below 1 GHz), our scheme could potentially be used for noninvasive [nuclear magnetic resonance] spectroscopy in structural biology (structure of proteins and nucleic acids) and in medical applications." This may potentially open up a whole new world of noninvasive imaging, where delicate biological samples could be analyzed using super-low radiation, nondestructive probe beams.

The new paper can be accessed ​in pre-publication, open-access form at arXiv.