
China’s claim it has ‘quantum’ radar may leave $17B F-35 naked - aerocapture
http://www.news.com.au/technology/innovation/inventions/chinas-claim-it-has-quantum-radar-may-leave-17-billion-f35-naked/news-story/207ac01ff3107d21a9f36e54b6f0fbab
======
pmontra
TLDR: Generate pairs of entangled photons. Split the pairs, send one photon to
the sky. Look at the photon at home to see what happened to the one in the sky
and what it run into.

~~~
tim333
Ah - don't think that'll work.

------
tropo
Stealth is not boolean.

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.

------
gus_massa
The article has a small skeptical part at the bottom, but this is nonsense. To
be more clear, I call bullshit.

> _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.

------
squozzer
I might end up doing the following --

[https://blogs.scientificamerican.com/critical-
opalescence/ho...](https://blogs.scientificamerican.com/critical-
opalescence/how-to-build-your-own-quantum-entanglement-experiment-
part-1-of-2/)

