# Question about double slit and entanglement

1. Mar 30, 2015

### jk22

i suppose an entangled source one arm is sent to a double slit the other to a polarizer.

I read in another thread that the entangled photon does not produce interferences through the slits.

What if i put on the other arm a polarizer before the first photon reach the slit? Because of entanglement the first photon will become polarized and hence produce an interference ?

2. Mar 30, 2015

### jfizzix

Is this the situation being considered?

You have highly entangled light, and you separate the photon pairs into arms A and B.
In arm A, you set up a double slit experiment
and in arm B, you do nothing.
At the end of A and the end of B, you have photon detectors that can record the spatial distribution of light hitting them.

You will usually see some interference, but the more entangled the light is, the less of it you'll see. For the sake of argument, we'll assume the light's perfectly entangled.

The reason that you won't see any interference in arm A is that if you consider only the light in arm A, it is incoherent.
The light in arm B, by itself is also incoherent.
However, if you look at the photon pairs (say by recording events when both detectors nearly click at the same time), you can see interference (say, by looking at the photons in A correlated to the photons in B which have a specific momentum range).
The state describing the pair of photons is coherent, so you can see interference, but only by looking at pairs of photons at a time.

No matter what you do in arm B, the statistics you measure in arm A, will be the same.
There is no way (theoretically or experimentally) of telling if a single particle is half of an entangled pair, if all you have is data on that single particle.
To detect entanglement, you need to also have data from the other particle it's allegedly entangled with.

Last edited: Mar 30, 2015
3. Mar 31, 2015