Quantum entangled-photon Chinese satellite.

[Peter Fairbrother]

On 04/08/16 13:33, Bastiani Fortress wrote:
> Quantum entanglement does not provide information passing faster than
> light, afaik. Either i misunderstood the news, or it's being falsely
> advertised.

>     [quote]
>     China to launch unbreakable quantum spy satellite - and it could one
>     day lead to a megascope the size of Earth that could 'spot a license
>     plate on Jupiter's moons'Satellite produces entangled photon pairs
>     which form an encryption key

It doesn't pass info faster than light, it "generates keys" using a 
slower-than-light side channel to agree which of two possible 
perpendicular orientations to test.


You can only test one orientation per photon, that's the physics part, 
testing in one orientation will destroy all information about its 
polarisation in a perpendicular orientation - and unless both ends test 
the same orientations for an entangled pair the test results will not 
match by spooky action at a distance.

One way to do this, though not very secure, is for both ends to preagree 
which orientations to test.

They can get the same information at the same, super-relativistic time, 
but it's a bit like DH, the information they get is random, no classical 
information is actually passed between ends.

Another method is for both sides to test orientations at random, then 
choosing pairs for which both ends chose to test the same orientation. 
This requires an authenticated, but not necessarily secret, side channel 
between ends.



Unless implemented vary carefully, either version can be mitm'd easily 
[fsvo "easily"] enough.

The first method can be mitm'd by creating photons with known (but not 
entangled) polarisations in pairs, if you know the prearranged 
orientations to create the photons in.


The second method is a little trickier to mitm, but the no-cloning 
theorem, which states you can't clone an arbitrary unknown quantum 
state, doesn't actually say that you can't clone a photon without 
testing it's polarisation in one orientation - it says that, for linear 
polarisation of entangled photons, you can't do it more than 2/3 of the 
time.

Then Mallory will get it right by chance half the time when cloning 
fails, so overall if Mallory tries to clone photons he will get the 
right result 5 times out of six.

This would roughly mean that the detectors would have to work correctly 
better than 5/6 of the time to prevent mitm - and single photon 
detectors which can do that, especially over orbital distances,  are 
like hen's teeth.

There are statistical methods which can work with less efficient 
detectors, but then the data rate drops to very slow indeed


-- Peter Fairbrother