Supercomputers with huge antenna arrays will not fit into aircraft. Aircraft radar will not fit into the small-diameter head of an air-to-air missile.
Becoming aware of a target is not the same as tracking a target.
Occasional detection is not the same as reliable detection.
China being able to do something against stealth does not mean that every other country can do the same thing.
BTW, there are motives to both hide and exaggerate military capability. Perhaps several countries have what China claims to have, but China itself doesn't have it.
> Quantum radar would send out bursts of photons while retaining their ‘pairs’. The changes in behaviour of the retained photon would then reveal what’s happening to the photon in the beam.
There is not visible change in the retained photons. You can't make any experiment that determine if they are entangled or not, and what happened with the other photon. The only options are:
1) Measure something (probably polarization) in the radar and measure something in the plane. Each measurement alone is boring, you only will get random data. It's only interesting if you synchronize and compare both measurements, because they will be correlated. This requires the cooperation from the plane, so it's not very probable.
2) Get the bounced photons, pair them with the photons in the radar, and make some measurement in both photons simultaneously. Sorting out which photon is the pair of which photon is unbelievable difficult, so this is even more impossible than the other options.
(This is possible in a tabletop experiment with lasers. LIGO does something somewhat similar to this. But the care to align and keep the mirror still is overwhelming. Anyway, you loose the quantum magic that makes it impossible to fake because you are using a lot of photons instead of one.)
3) Get the bounced photons, make some measurement in the stream of bounced photons and do a similar measurement in the stream of the photons that didn't fly. Now you can compare both and try to synchronize them. This has a extremely tiny probability of veracity.
It's possible to do this in a tabletop experiment, with a very dark background, with carefully aligned optic to avoid the synchronization problem, with a lot of damping and very still optic support for the same reason. And very fast electronic to detect the coincidences.
You can use that splitting crystal can create two very synchronized stream of photons, and keep one and bounce the other against the plane and hopefully try to guess the delay necessary to resynchronize them. It may work in a very good conditions like a tabletop experiment. Using it in the wild is almost impossible, because the bounced photons will be mixed with a lot of other photons from the ambient, and the signal will twinkle like a star due to alignment, temperature changes, ...
And that will not use the quantum entanglement part of the signal, it will be only be only a classical synchronization of two signal. You can try to measure the polarization of the bounced photons and the original photons to try to measure the entanglement, but keeping the polarization after traveling and bouncing and whatever looks almost impossible.
https://blogs.scientificamerican.com/critical-opalescence/ho...
