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?