Is it possible to determine if a photon is entangled?

[ACG]

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

 

[peter0302]

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.

 

[denian]

Hi ACG,

Thank you for your question. The answer is both yes and no. Let me explain.

First, let's define what entanglement is. Entanglement is a phenomenon in quantum mechanics where two or more particles are connected in such a way that the state of one particle is dependent on the state of the other, even if they are separated by a large distance. This means that if you measure the state of one particle, you can determine the state of the other particle without directly measuring it. This is what makes entanglement such a fascinating and useful concept in quantum mechanics.

Now, to answer your question, yes, it is possible to determine if a photon is entangled with another photon. This can be done by performing a measurement on the photon and comparing the results with the expected outcomes based on the entangled state. If the results match, then the photon is entangled.

However, your proposed method of using the EPR paradox for faster-than-light communication is not possible. This is because measuring the state of a photon will destroy the entanglement between it and the other photon. So, even if Alice and Bob had a large number of entangled photon pairs, by measuring just one photon, they would destroy the entanglement of that pair and would not be able to use it for communication.

Furthermore, the state of a photon cannot be used to transmit information. This is because the state of a photon is probabilistic in nature and can only be determined with a certain probability. So, even if Bob tried to use the state of the photon to determine if it was entangled or not, he would only have a probability of being correct. This makes it impossible to use the state of a photon for reliable communication.

In conclusion, while it is possible to determine if a photon is entangled, it cannot be used for faster-than-light communication as the entanglement is destroyed during the measurement process. Additionally, the probabilistic nature of quantum mechanics makes it impossible to use the state of a photon for reliable communication. I hope this helps clarify your thinking.