- #1
greypilgrim
- 515
- 36
Hi.
As far as I understand the Franson interferometer, the photons are in an entangled state like
$$\left|\Psi\right\rangle=\frac{1}{\sqrt{2}}\left(\left|\text{short}\right\rangle\left|\text{short}\right\rangle+\left|\text{long}\right\rangle\left|\text{long}\right\rangle\right)$$
if the setup is symmetrical.
This kind of entanglement is new to me. For example entangled polarization or spins are created at the source, while above state is written in a basis that depends on the setup itself, which might be far away from the source. What happens if we remove the setup altogether after the photons have been emitted, but haven't entered the interferometer yet? Above expansion wouldn't make sense anymore. What is now entangled about the photons?
A second question: I read that there will be events where one photons takes the short and the other the long path. As this can clearly be detected by comparing the arrival times, it doesn't create interference, those measurements are excluded in postselection. Apparently this discards half of all events. But from the above state I don't see how this could even happen. Or does the source also emit different states that lead to this behaviour?
EDIT: Well now I'm completely confused. If we discard all events where the photons don't arrive at the same time, the ones remaining must obviously be short/short and long/long. What's quantum about this?
As far as I understand the Franson interferometer, the photons are in an entangled state like
$$\left|\Psi\right\rangle=\frac{1}{\sqrt{2}}\left(\left|\text{short}\right\rangle\left|\text{short}\right\rangle+\left|\text{long}\right\rangle\left|\text{long}\right\rangle\right)$$
if the setup is symmetrical.
This kind of entanglement is new to me. For example entangled polarization or spins are created at the source, while above state is written in a basis that depends on the setup itself, which might be far away from the source. What happens if we remove the setup altogether after the photons have been emitted, but haven't entered the interferometer yet? Above expansion wouldn't make sense anymore. What is now entangled about the photons?
A second question: I read that there will be events where one photons takes the short and the other the long path. As this can clearly be detected by comparing the arrival times, it doesn't create interference, those measurements are excluded in postselection. Apparently this discards half of all events. But from the above state I don't see how this could even happen. Or does the source also emit different states that lead to this behaviour?
EDIT: Well now I'm completely confused. If we discard all events where the photons don't arrive at the same time, the ones remaining must obviously be short/short and long/long. What's quantum about this?
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