FTL Communication-Best Candidate?

rq704c

What do you mean the coincident measurments cannot carry ftl information. This is due to the experiemental setup of the crystal location correct, and not because it's not possible?

Cthugha

No, coincidence counting means you have to compare data from two spatially separated detectors. So you have to send the data from A to B to get some information. This data cannot be transferred faster than light, therefore there is no possibility for ftl information transfer by means of experiments, which require coincidence counting.

rq704c

If you have two momentum entangled photons, one heading down axis "a" and the other down axis "b" and you destroy all the axis "a" photons by measuring their position, all the axis "b" photons should not interefere with themsselves because you have forced them into a postion definite state. Setting up a double slit on axis b and observing no intereference would be a good indication the other photons are being measured given most of the photons are being down converted and are entangled.

Cthugha

The flaw in this scheme is exactly, what I explained in my last post. See page 46 of the Dopfer thesis for a detailed explanation. To cut it short single photon interferences and coincidence count interferences are complementary. If you have coincidence count interferences, you can do position measurements, but there is no single photon interference pattern to destroy because the light source in this single arm is too incoherent to produce an interference pattern.

On the other hand, if there is a single photon interference pattern in one arm, this means automatically, that the momentum entanglement is already destroyed and the two photons behave like independent photons. You can imagine this as happening due to the strong decrease in possible directions (or k-vectors) for the photon as you increase the distance between the crystal and the double slit. As mentioned before this decrease destroys the coincidence count interference pattern, which is a measure of the degree of entanglement.

This is not a consequence of the geometry, but a general problem. You want to have as few k-vectors as possible to enlarge coherence, but you need many k-vectors to achieve meaningful momentum entanglement.

The references 23-25 in the Dopfer thesis also discuss this problem a bit.

GRB 080319B

I think this wiki page addresses your questions.

Quote from Wikipedia: "Science fiction style space travel, dubbed "True" FTL, in which matter exceeds the speed of light in its own frame of reference, defies known physics. On the other hand, what some physicists refer to as "apparent" or "effective" FTL[1][2][3][4] is the hypothesis that unusually distorted regions of spacetime might permit matter to reach distant locations faster than what it would take light in the "normal" route (though still moving subluminally through the distorted region). Apparent FTL is not excluded by general relativity. Examples of apparent FTL proposals are the Alcubierre drive and the traversable wormholes, although the physical plausibility of these solutions is uncertain."