Is it possible to determine if a photon is entangled?

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ACG
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Hi! This may be a stupid question, but here goes.

Is it possible to do a measurement to determine if a photon's waveform is entangled with another photon? You don't need to know who it's entangled with, and it's all right for the measurement to disrupt the entanglement.

If this is the case, I don't see how you can't use the EPR paradox for FTL communication. Alice and Bob have a bunch of entangled photon pairs and Alice wants to transmit some information: say, the bits 0101.

All Alice does is measure photons 2 and 4. This causes photons 2 and 4 to drop out of entanglement for both parties. Alice and Bob will now have two entangled photons and two "normal" photons.

Bob now uses the entanglement measurement to determine if his four photons are entangled. He defines a photon which was entangled (and is no longer) to be a 1 and a still-entangled photon to be a 0. After the observation, no photons are entangled.

What's wrong with my thinking? There's got to be something wrong here. The only thing I can think of would be that you can't determine if a photon's wave state is entangled by looking at it because the entanglement is destroyed during the process of making the measurement.

Thanks in advance,

ACG
 

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  • #2
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Photons that are momentum-entangled with another will not make the same interference pattern in a double slit that normal photons do. This is one of the reasons FTL communication is impossible using entangled photons. So that would be one way of determining if photons are entangled with another.

And, "breaking" the entanglement does nothing to the twin that can be observed without comparing both. Another reason you can't use it for FTL.
 

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