Could quantum radars expose stealth planes?

Radar technology has become an ever more important tool in warfare since the Second World War, used to monitor the electromagnetic spectrum to detect and track enemy aircraft, missiles, satellites and other systems that in turn have grown more sophisticated in evading detection.

Building better, more sensitive and harder-to-detect radar systems remains a key focus for defence manufacturers today, especially since in October 2018 the US Navy designated the electromagnetic spectrum as a warfighting domain on par with sea, land, air, space and cyber.

Last year, China’s biggest defence electronics company, state-owned China Electronics Technology Group, announced it had developed a next-generation “quantum radar system” that, it claimed, can detect ballistic missiles and other objects flying “at high speed through space”. Two years previously, the group said it had tested a quantum radar to a range of 100km (60 miles).

The expectation is that quantum radar, due to the unique behaviour of quantum entanglement, could, among other things, “un-stealth” aircraft by detecting objects with a greater level of accuracy than conventional radar. Therefore, if China’s announcement is true - and it hasn’t provided any evidence to back its claim - it would be a huge win for both its defence and quantum capabilities.

China is keen to become both a military super power and a leader in quantum technology, having ring-fenced billions towards research and development. But it is not the only runner in the race.

According to Ned Allen, the chief scientist at US defence technology company Lockheed Martin, the idea for a quantum radar was first floated at a meeting between him, his assistant and a professor of quantum information science at the University of Southern California (USC) in 2002 or thereabouts.

They decided to suggest it to the strategic technology office of the US Defense Advanced Research Projects Agency (Darpa), which commissioned a one-year research project. Both the US and Lockheed-Martin have been working on the technological development of quantum radar ever since - though separately as, ironically, Darpa didn’t commission Lockheed to run the subsequent quantum-sensing programme that spun from the USC team’s initial research.

Canada has also invested C$2.7m (£1.93m) into developing quantum radar via an ongoing research project at the University of Waterloo. The UK, through Innovate UK, is funding an ongoing study by Qinetiq into the feasibility of using quantum metrology in radar and lidar systems.

Quantum radar technology is based on the principles of quantum illumination and quantum entanglement, though there are other techniques being developed that are not properly quantum. Lockheed is doing experimental work developing a classical radar incorporating components that use quantum principles, for example.

A classical radar works by sending out a directional beam of radiation at radio or microwave frequencies, and through a receiver detecting reflections from any objects in the path of the signal, in order to calculate their location and speed.

A quantum radar, in simple terms, uses a source of entangled photons that are strongly correlated and have the same inseparable identity and experiences and work as one quantum system, even when separated. To detect an object at distance the beam of photons is split, with one half transmitted and the other retained at the basestation for comparison with the reflections.

Sending out entangled quanta or photons instead of classical radiation, in theory, offers several advantages. The first is ‘better’ image resolution without an increase in frequency.

“The resolution from any visualisation device is directly proportional to the energy of the photons you use to identify the device, and one of the most interesting things about quantum radar is the behaviour of the radar beam as it is propagated through the atmosphere, because the character of the image beamed back is a function of all the photons added together,” explains Allen.

This means it is possible to get a much higher resolution of targets than classical radar but at the same frequency, even if they have been physically minimised by stealth techniques.

“There is much ongoing work to raise the frequency of radar, but quantum allows you to do that much more easily and more elegantly,” adds Allen.

Furthermore, with entangled photons it should be easier to separate background noise from what is actually being reflected back off an object, says Jonathan Baugh, an associate professor at the University of Waterlooís Institute for Quantum Computing (IQC) and the Department of Chemistry, who is leading a quantum radar research project with three other researchers at IQC and the Waterloo Institute for Nanotechnology.

Radar frequencies sometimes suffer because of lots of background noise due to thermal and black body radiation, stray radio signals, or radio noise from things such as solar wind hitting the atmosphere, which can obscure the signal.

This is particularly acute in higher altitudes like the Arctic regions. In areas like this, stealth aircraft, which do not reflect back much light, just become part of the background noise.

By using a split beam of entangled photons, quantum radar theoretically will allow militaries to boost the radar signal and discard some of the noise, making detection more effective.

For example, if an emitted photon came into contact with a target it would be reflected off it and return to the station. At this point it is possible to perform a ‘correlation experiment’ or measurement on both photons to determine if it is in fact the returning photon and not just background noise.

“This gives you an advantage because then it’s possible to separate out the ‘real’ photons - the ones known to be sent from the basestation and reflected back - from ambient background noise. That information is useful as it allows you to sift out a small signal buried in noise and can tell you the photon you are detecting is not just random,” Baugh explains.

It is part of the “weirdness” of quantum entanglement, says Seth Lloyd, a professor of mechanical engineering and physics at the Massachusetts Institute of Technology, that makes it possible to measure and detect the photon sent out, even in extremely loud and noisy environments.

“In those situations, theoretically quantum radar will allow militaries to boost the signal and discard some of the noise, making detection more effective,” adds Lloyd.

Furthermore, because quantum gets better results from less power, it would be harder to detect by the enemy.

“If planes detect enemy radar they can put jamming on, but it would be a lot harder to detect quantum radar - I think that is one of the main implications,” says Lloyd.

Despite China’s claims to have developed a quantum radar system - which Baugh, Lloyd and Allen dismiss as “not credible” - there are still many technological challenges to realising such a system.

The main one is that quantum information is very fragile. “It can be immediately or completely destroyed by the slightest bit of noise or atmosphere disturbance, which is known as decoherence and which destroys its utility,” says Allen.

Lockheed, he says, is working to overcome this problem by encasing the quantum photons in a cocoon, which he fondly refers to as “Cinderella’s carriage”.

Allen say he believes the Chinese have conducted a successful experiment using a quantum radar system, but that they overcame the decoherence problem by effectively cheating. Chinese researchers, he says, mounted the quantum radar generator on a satellite in space, which essentially means it is not propagating the quantum beam through the atmosphere, but a vacuum. Plus, the receiver was on a high-altitude mountain in Tibet.

“Though they lost 900 million out of 910 milllion photons, ten [million] got through out of chance because they cranked up the power and reduced the atmosphere,” he explains.

Other challenges to developing quantum radars include creating highly reliable streams of entangled photons and building extremely sensitive detectors.

“Because we are working at the single photon level, we are dealing with power levels that are many order of magnitudes weaker than conventional radar systems, which creates a technical challenge,” explains Baugh.

Baugh and a team of researchers at the University of Waterloo are developing a reliable source of quantum light to create pairs of photons at a very high rate to build up enough signal to detect things in real time, he explains.

The best entangled photon sources currently generate around 10 million pairs per second using parametric down-conversion. This is an optically pumped technique using a laser and shining it into a non-linear crystal to randomly produce entangled photons.

The researchers are building an electrical device that they hope will demonstrate a billion per second or one pair every nanosecond via an electrically driven source that would not be random but controlled.

The new entangled photon source would be directly plugged into a quantum lidar system; it will emit photons in the visible or near infrared regime - or, with wavelength conversion to microwave frequencies, it can be used for quantum radar. With another year and a half to go, Baugh says experiments so far are meeting expectations.

There are other groups, one led by Chris Wilson also at the University of Waterloo, creating entangled photon pairs directly in the microwave region.

Another challenge is that the technology will need to work at 4 degrees Kelvin, which is typical for most solid-state quantum technologies to access the quantum states exploited.

And, in practice, it will need to operate in conjunction with what Baugh describes as “really complicated real-time analysis” to make it a feasible system to use, but he adds, “I am optimistic it will be useful”.

That quantum radar, once developed, would ‘unstealth stealth planes’ is only a theory, says Lloyd, and it’s more likely that an element of stealithiness would remain.

Allen agrees that to say quantum radar will defeat stealth is a “gross over-simplification of the issue”.

“The main benefit of quantum radar, if you had one that worked, would be higher resolution of the image, a much more precise image, which would allow you to detect not only the existence of an intrusive enemy airplane or missile but would allow you to determine its shape, speed and size - how many fins it has and so forth. Whereas with a regular radar you just see a blob and you don’t know what it is,” he says.

As for the race to claim quantum radar capabilities, neither Lloyd nor Allen think China is close to posing a threat.

“This is not an area China has typically excelled in, so I wouldn’t say they are making leaps and bounds; however, if they spend US$9bn they might get something done,” says Lloyd.

He adds that some in the US defence departments he has spoken to feel the Chinese announcement was bluff to spook Americans and their use of stealth fighters.

However, according to Allen: “If the Chinese were successful in solving the decoherence problem, it could make many quantum technologies”.

In fact, in the race for quantum supremacy, the US, Canada, UK and others have an obvious advantage - the ability to tap into the work of their allies.

Will anyone, realistically, be able to build a commercial quantum radar and if so, when? Lloyd, who produced one of the first papers on quantum illumination, says that while a large-scale quantum computer might be “a decade off”, a prototype quantum radar working in the field could be merely “five to ten years” away.

“There is still some fundamental science to be done, but in a couple of years it will no doubt have more applied engineering that can turn it into a real technology - all contingent on funding,” he concludes.