In the 1970s, at the height of the Cold War, American military planners began to worry about the threat to US warplanes posed by new, radar-guided missile defenses in the USSR and other nations. In response, engineers at places like US defense giant Lockheed Martin’s famous “Skunk Works” stepped up work on stealth technology that could shield aircraft from the prying eyes of enemy radar.

The innovations that resulted include unusual shapes that deflect radar waves—like the US B-2 bomber’s “flying wing” design (above)—as well as carbon-based materials and novel paints. Stealth technology isn’t yet a Harry Potter–like invisibility cloak: even today’s most advanced warplanes still reflect some radar waves. But these signals are so small and faint they get lost in background noise, allowing the aircraft to pass unnoticed.

China and Russia have since gotten stealth aircraft of their own, but America’s are still better. They have given the US the advantage in launching surprise attacks in campaigns like the war in Iraq that began in 2003.

This advantage is now under threat. In November 2018, China Electronics Technology Group Corporation (CETC), China’s biggest defense electronics company, unveiled a prototype radar that it claims can detect stealth aircraft in flight. The radar uses some of the exotic phenomena of quantum physics to help reveal planes’ locations.

It’s just one of several quantum-inspired technologies that could change the face of warfare. As well as unstealthing aircraft, they could bolster the security of battlefield communications and affect the ability of submarines to navigate the oceans undetected. The pursuit of these technologies is triggering a new arms race between the US and China, which sees the emerging quantum era as a once-in-a-lifetime opportunity to gain the edge over its rival in military tech.

Stealth spotter

How quickly quantum advances will influence military power will depend on the work of researchers like Jonathan Baugh. A professor at the University of Waterloo in Canada, Baugh is working on a device that’s part of a bigger project to develop quantum radar. Its intended users: stations in the Arctic run by the North American Aerospace Defense Command, or NORAD, a joint US-Canadian organization.

Baugh’s machine generates pairs of photons that are “entangled”—a phenomenon that means the particles of light share a single quantum state. A change in one photon immediately influences the state of the other, even if they are separated by vast distances.

Quantum radar operates by taking one photon from every pair generated and firing it out in a microwave beam. The other photon from each pair is held back inside the radar system.

Equipment from a prototype quantum radar system made by China Electronics Technology Group Corporation Imaginechina via AP Images

Only a few of the photons sent out will be reflected back if they hit a stealth aircraft. A conventional radar wouldn’t be able to distinguish these returning photons from the mass of other incoming ones created by natural phenomena—or by radar-jamming devices. But a quantum radar can check for evidence that incoming photons are entangled with the ones held back. Any that are must have originated at the radar station. This enables it to detect even the faintest of return signals in a mass of background noise.

Baugh cautions that there are still big engineering challenges. These include developing highly reliable streams of entangled photons and building extremely sensitive detectors. It’s hard to know if CETC, which already claimed in 2016 that its radar could detect objects up to 100 kilometers (62 miles) away, has solved these challenges; it’s keeping the technical details of its prototype a secret.

Seth Lloyd, an MIT professor who developed the theory underpinning quantum radar, says that in the absence of hard evidence, he’s skeptical of the Chinese company’s claims. But, he adds, the potential of quantum radar isn’t in doubt. When a fully functioning device is finally deployed, it will mark the beginning of the end of the stealth era.

China’s ambitions

CETC’s work is part of a long-term effort by China to turn itself into a world leader in quantum technology. The country is providing generous funding for new quantum research centers at universities and building a national research center for quantum science that’s slated to open in 2020. It’s already leaped ahead of the US in registering patents in quantum communications and cryptography (see chart).

A study of China’s quantum strategy published in September 2018 by the Center for a New American Security (CNAS), a US think tank, noted that the Chinese People’s Liberation Army (PLA) is recruiting quantum specialists, and that big defense companies like China Shipbuilding Industry Corporation (CSIC) are setting up joint quantum labs at universities. Working out exactly which projects have a military element to them is hard, though. “There’s a degree of opacity and ambiguity here, and some of that may be deliberate,” says Elsa Kania, a coauthor of the CNAS study.

China’s efforts are ramping up just as fears are growing that the US military is losing its competitive edge. A commission tasked by Congress to review the Trump administration’s defense strategy issued a report in November 2018 warning that the US margin of superiority “is profoundly diminished in key areas” and called for more investment in new battlefield technologies.

One of those technologies is likely to be quantum communication networks. Chinese researchers have already built a satellite that can send quantum-encrypted messages between distant locations, as well as a terrestrial network that stretches between Beijing and Shanghai. Both projects were developed by scientific researchers, but the know-how and infrastructure could easily be adapted for military use.

The networks rely on an approach known as quantum key distribution (QKD). Messages are encoded in the form of classical bits, and the cryptographic keys needed to decode them are sent as quantum bits, or qubits. These qubits are typically photons that can travel easily across fiber-optic networks or through the atmosphere. If an enemy tries to intercept and read the qubits, this immediately destroys their delicate quantum state, wiping out the information they carry and leaving a telltale sign of an intrusion.

QKD technology isn’t totally secure yet. Long ground networks require way stations similar to the repeaters that boost signals along an ordinary data cable. At these stations, the keys are decoded into classical form before being re-encoded in a quantum form and sent to the next station. While the keys are in classical form, an enemy could hack in and copy them undetected.

To overcome this issue, a team of researchers at the US Army Research Laboratory in Adelphi, Maryland, is working on an approach called quantum teleportation. This involves using entanglement to transfer data between a qubit held by a sender and another held by a receiver, using what amounts to a kind of virtual, one-time-only quantum data cable. (There’s a more detailed description here.)

Michael Brodsky, one of the researchers, says he and his colleagues have been working on a number of technical challenges, including finding ways to ensure that the qubits’ delicate quantum state isn’t disrupted during transmission through fiber-optic networks. The technology is still confined to a lab, but the team says it’s now robust enough to be tested outside. “The racks can be put on trucks, and the trucks can be moved to the field,” explains Brodsky.

It may not be long before China is testing its own quantum teleportation system. Researchers are already building the fiber-optic network for one that will stretch from the city of Zhuhai, near Macau, to some islands in Hong Kong.

Quantum compass

Researchers are also exploring using quantum approaches to deliver more accurate and foolproof navigation tools to the military. US aircraft and naval vessels already rely on precise atomic clocks to help keep track of where they are. But they also count on signals from the Global Positioning System (GPS), a network of satellites orbiting Earth. This poses a risk because an enemy could falsify, or “spoof,” GPS signals—or jam them altogether.

Lockheed Martin thinks American sailors could use a quantum compass based on microscopic synthetic diamonds with atomic flaws known as nitrogen-vacancy centers, or NV centers. These quantum defects in the diamond lattice can be harnessed to form an extremely accurate magnetometer. Shining a laser on diamonds with NV centers makes them emit light at an intensity that varies according to the surrounding magnetic field.

Wikimedia Commons

Ned Allen, Lockheed’s chief scientist, says the magnetometer is great at detecting magnetic anomalies—distinctive variations in Earth’s magnetic field caused by magnetic deposits or rock formations. There are already detailed maps of these anomalies made by satellite and terrestrial surveys. By comparing anomalies detected using the magnetometer against these maps, navigators can determine where they are. Because the magnetometer also indicates the orientation of magnetic fields, ships and submarines can use them to work out which direction they are heading.

China’s military is clearly worried about threats to its own version of GPS, known as BeiDou. Research into quantum navigation and sensing technology is under way at various institutes across the country, according to the CNAS report.

As well as being used for navigation, magnetometers can also detect and track the movement of large metallic objects, like submarines, by fluctuations they cause in local magnetic fields. Because they are very sensitive, the magnetometers are easily disrupted by background noise, so for now they are used for detection only at very short distances. But last year, the Chinese Academy of Sciences let slip that some Chinese researchers had found a way to compensate for this using quantum technology. That might mean the devices could be used in the future to spot submarines at much longer ranges.

A tight race

It’s still early days for militaries' use of quantum technologies. There’s no guarantee they will work well at scale, or in conflict situations where absolute reliability is essential. But if they do succeed, quantum encryption and quantum radar could make a particularly big impact. Code-breaking and radar helped change the course of World War II. Quantum communications could make stealing secret messages much harder, or impossible. Quantum radar would render stealth planes as visible as ordinary ones. Both things would be game-changing.

It’s also too early to tell whether it will be China or the US that comes out on top in the quantum arms race—or whether it will lead to a Cold War–style stalemate. But the money China is pouring into quantum research is a sign of how determined it is to take the lead.

China has also managed to cultivate close working relationships between government research institutes, universities, and companies like CSIC and CETC. The US, by comparison, has only just passed legislation to create a national plan for coordinating public and private efforts. The delay in adopting such an approach has led to a lot of siloed projects and could slow the development of useful military applications. “We’re trying to get the research community to take more of a systems approach,” says Brodsky, the US army quantum expert.

Still, the US military does have some distinct advantages over the PLA. The Department of Defense has been investing in quantum research for a very long time, as have US spy agencies. The knowledge generated helps explains why US companies lead in areas like the development of powerful quantum computers, which harness entangled qubits to generate immense amounts of processing power.

The American military can also tap into work being done by its allies and by a vibrant academic research community at home. Baugh’s radar research, for instance, is funded by the Canadian government, and the US is planning a joint research initiative with its closest military partners—Canada, the UK, Australia, and New Zealand—in areas like quantum navigation.

All this has given the US has a head start in the quantum arms race. But China’s impressive effort to turbocharge quantum research means the gap between them is closing fast.