How to know if two photons are entangled?

[Dorilian]

If I give you a black box and two photons go out of it (not necessarily in opposite directions), how can we know if the photons are entangled?

 

[A. Neumaier]

If I give you a black box and two photons go out of it (not necessarily in opposite directions), how can we know if the photons are entangled?

One can't. But if the box produces _many_ pairs of photons, one can calculate statistics and decide on the basis of that.

 

[Dorilian]

One can't. But if the box produces _many_ pairs of photons, one can calculate statistics and decide on the basis of that.

Ok, so, we need to deduce what kind of entanglement we have to do the experiments. Thanks.