How are entangled particles separated/distinguished?

[San K]

Entangled particles are used in various experiments for example:

DCQE (delayed choice quantum eraser)
Bells tests
Mach Zehnder (modified)
DCES (delayed choice entanglement swapping - Ma)

Now, one out of a trillion photons, on average, gets entangled after passing through the SPDC (?)

I can think of, at the least, four kinds of detections:

1. Photons which are not entangled and don't have opposite signs and fall in the same time bin
2. Photons which are not entangled but do have opposite signs and fall in the same time bin
3. Photons which are entangled (and obviously have opposite spins?)...this is the one in a trillion photon?
4. Photons which are noise (generated not necessarily from the experiement but from the nearby enviroment) and that fall in the same time bin with opposite signs.

Questions:

A. Is my understanding of the experiment correct?
B. how are the above 4 distinguished? Partial Answer:

for 1 its easy --- they don't seem correlated.
for 4 literature tells us that we cannot do anything about them...if they fall within the same time bin (i.e. within the temporal resolution of the co-incidence counter)

How do we distinguish between 2 & 3 above?

 

[DrChinese]

You cannot distinguish them. However, the ones of categories 1 and 2 are going to be very rare. The reason is that the average time between clicks is perhaps 1-5 microseconds. That is about 100 times larger than the coincidence window (depending on where you set it, I think Weihs et al used 6 ns). So the "random" unentangled photons must BOTH appear within the same window. You might roughly estimate the chances of that happening as being around (100^-2)(some intensity factor).

Keep in mind that truly random unpaired photons should not appear at all. To be collected, they come out at certain angles from the PDC crystal. And filters keep out light of the wrong wavelength. So I would speculate that most would come from paired photons in which one had de-cohered.

 

[San K]

You cannot distinguish them. However, the ones of categories 1 and 2 are going to be very rare.

thanks for the information DrChinese

The reason is that the average time between clicks is perhaps 1-5 microseconds. That is about 100 times larger than the coincidence window (depending on where you set it, I think Weihs et al used 6 ns). So the "random" unentangled photons must BOTH appear within the same window. You might roughly estimate the chances of that happening as being around (100^-2)(some intensity factor).

got it, thanks

Keep in mind that truly random unpaired photons should not appear at all.

what's a truly random unpaired photon?

i expected you would have said ...truly random paired photon...paired as in opposite signs(?)