A vehicle-mounted warning radar system built for the Chinese Army (Picture: Chinese Ministry of Defence).
By David Hambling, technology expert
Before the Second World War, the only way to detect incoming aircraft was the Mk 1 Eyeball, useless in the dark or bad weather, or with short-range listening apparatus.
Radar changed the situation dramatically. Things are set to change again with the new technology of quantum radar.
Radar works something like a searchlight. The radar scans the sky with a beam of radio waves, and any reflections – say, from, an aircraft - return to the dish where they are detected.
Radar was the secret weapon that helped win the Battle of Britain, spotting formations of Luftwaffe aircraft at long range in the dark, so fighter squadrons could be dispatched to intercept them.
Ever since then, a large part of air warfare has been devoted to avoiding or neutralising radar.
In the post-war period, bombers started flying close to the ground, hiding behind the terrain and staying under the coverage of radar.
This is why RAF jets may be found hugging valley floors in designated ‘low flying areas’ in sparsely populated areas.
However, flying low is less use against modern opponents with airborne radar.
Other means of dealing with radar were developed, including electronic jamming - broadcasting radio noise on the same frequency as the radar to blank out the returns from the aircraft - and electronic spoofing - sending signals which create fake radar returns from non-existent aircraft.
Some modern aircraft like the F-35 also have stealth: a combination of shaping and materials to make their radar reflection so small it gets lost in the background noise.
Finally, attackers may simply bomb the radar sites, which are easy to spot from their powerful radio emissions.
This combination of measures allows combat operations over defended territory, but that may change with the introduction of quantum radar.
A quantum radar produces two radio beams, which are connected by so-called “quantum entanglement”.
This is a peculiar effect in which two photons in different beams can be connected with each other, even though they are miles apart.
It is a weird effect; even Einstein was doubtful about whether quantum entanglement was possible, calling it “spooky action at a distance,” but since then it has been proven to work.
One beam is sent out, just like a standard radar beam, and bounces back off objects in the sky. The second, ‘idler’ beam, remains inside the system.
Any reflections which come back can be matched with those in the idler beam.
Entanglement means that any stray radio waves, or those that came from any other sources, such as spoofing or jamming, can be filtered out.
Background noise is also eliminated, so even the faint reflections from a stealth aircraft can be spotted relatively easily.
This means quantum radar can, in theory, beat current methods of countering radar.
An added benefit is that, because of their low power levels, quantum radar is difficult to detect, making it less vulnerable to enemy action.
The theory may be good, but getting quantum radar to work in practice is challenging.
There are active quantum radar programs in the US, China, Russia and Canada, but researchers have always spoken about it as something which is in the very early stages of laboratory development – until now.
A typical program is the Canadian effort, focusing on a long-term replacement for their radar network inside the Arctic circle.
"We're at a very early stage, and this investment is about seeing whether we can really turn theory into practice," one researcher told the BBC.
However, last month Chinese company CETC showed off what they claimed to be a prototype of a quantum radar system.
No demonstration was given, and full briefings were only given to Chinese journalists, but the company does have a track record in this field.
It is possible that what was displayed was just a mock-up, an empty box rather than a working system, but it is also possible that the Chinese have leapt ahead in this area.
Does that mean it’s time to get worried? Not necessarily, according to Justin Bronk of defence think-tank RUSI.
He says detecting a plane is not the same as shooting it down:
“It is important to state that detection is only the first step in the kill chain required to engage an aircraft.
“There is no guarantee that quantum radar could produce target-grade tracking data to enable an intercept, even if it can detect stealth aircraft at significant distances.”
Quantum radar remains unproven technology.
We cannot yet tell whether it will work well enough to track actual aircraft accurately in real time.
But if it does work as well as the developers hope, it could shift the balance of air warfare again, handing a decisive advantage to the defenders just as the original radar did in the Second World War.