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