It is a well-known 'principle' in physics that only the two-way speed of light can be measured. Any experiment you try to construct to measure the speed of light only in one direction results in a return light signal required to synchronize clocks. So it seems impossible to truly have a one-way speed-of-light measurement.
However, can entangled photons be used in a clever way to make a one-way measurement? If so, how?
One possibility might be to create an entangled pair, directing one photon to a Faraday rotator that is a known distance away. The state of the second photon can then be measured. Then by doing many repeated measurements one might be able to built up a statistical picture of the average time of arrival of the first photon to the Faraday rotator.
Assuming this did work, would it actually 'count' as a one-way measurement or have we in fact 'cheated' by using a photon pair in the first place?
Hi Derek,
Interesting question.
In the first case, I don't think that you can do what you've suggested. The photon you keep will of course give correlated measurements with the distant photon, but without including a classical channel it won't tell you when it was measured (see for example Gisin's work on speed of entanglement).
There are potentially some alternatives, however. The first to look at is the clock synchronisation protocol of Josza et al., http://arxiv.org/pdf/quant-ph/0004105v3.pdf, where entanglement provides an improvement in the synchronisation. Once you have clock synchronisation and known distance, then a protocol to determine the speed of light in the one-way transmission should be straightforward based on photon arrival times.
I suppose another point is that what we've been discussing doesn't directly measure the speed of light. We've inferred the speed of light from a determination of the propagation time and the known distance. I wonder if we instead asked questions around the Doppler shift (for example) if we might not make a more meaningful headway. So I'm wondering about a configuration where an oscillating source of entangled photons is sent to a receiver, which tries to replicate the signal and return it. The Doppler shift in each direction must go like the speed of light in each direction. Conventionally we would expect these two speeds to be the same, but if the space shows time reversal symmetry breaking and the two speeds are therefore different, then there should be a difference in the round trip Doppler signal. This in turn would be a measure for the differences in the speeds.
Does this work?
Andy
Let me dispute the claim you make, though it is indeed one that is made often: *any* measurement of a one-way velocity requires a synchronisation procedure for the two clocks. That is, in order to measure the time the system spends in going from clock A to clock B, you take the difference of the time the clock A marks at departure and the time the clock B marks on arrival. But this only works if the two clocks are synchronised. But there are several ways to synchronise clocks.
If we use Einstein's way, then it is a matter of definition that the one way speed of light is always constant (the fact that the two way speed of light is constant being here assumed).
Experiments on the one way speed of light are not impossible, but they must use, implicitly or explicitly, a different synchronisation procedure than Einstein's. Slow clock transport is possible: synchronise two clocks at the same point, and then carry them slowly apart. However, if relativity is correct, then essentially all synchronisation procedures are equivalent. Otherwise, if there is an ether, then one-way speed of light measurements could be done with slow clock synchronisation and might show discrepancies. In fact, the fact that slow clock transport agrees well wit Einstein synchronisation (experimentally observed) gives bounds on possible violations of relativity.
"in all these experiments the information speed is always c" said John Gideon....
Is that true? Information speed is that speed which fulfills change of information between the two entangled photons. Theoretically this speed should be differentiated from speed of light.
We don't know the speed of information. Some physicist say that the change of Information between two entangled photons is simultaneously.
But that are theoretical thoughts, I have no possibility to show it in physical way....
John said: "But one cannot measure the one-way speed of light, therefore [...]"
This "cannot" which is not justified sounds like a ban or paralysis caused by Einstein. He told us: "If you try to move your clocks they will break apart or dilate". No! They don't if they are constructed properly.
Just remember that impressing piece of history of science when John Harrison solved the problem of how to determine longitude. Even then the Einsteinians would have shouted: "This is impossible!".
Is is not, despite it has been proved to be.
One way speed of light can be measured comfortably using two clocks of course the experimental problems of remote clock stability influences of gravity etc can skew results, but should be able to demonstrate variation. The key is the developed concept, not even practical means of the absolute simultaneity. This however, cannot be demonstrated using Lorentz transformation alone.
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