I Can the quantum eraser work without coincidence?

[Erik Ayer]

I understand that downconverted light is not coherent, so that there are essentially many overlapping interference patterns and that coincidence detection filters out one of those patterns. However, could the downconverted light be filtered such that interference can be detected without coincidence? I imaging this being done with something like a long tube that absorbs light going off at angles rather than straight through the tube - it would only allow light in the polarization-entangled parts of the intersecting degenerate cones through.

This would, of course, screw up the momentum entanglement. However, the entanglement used in the quantum eraser is polarization, which should not be affected by the spatial filtering. I'm sure gobs of people have thought of this before me but I haven't run across any sort of discussion on it.

 

[azntoon]

Does anyone have any insight?The short answer is that it is not possible to filter downconverted light in such a way that interference can be detected without coincidence. The reason for this is that downconverted light does not exist in an interference state until it is interfered with another light beam. In other words, the interference pattern in downconverted light is generated by the overlap of two beams. This means that in order to detect interference from downconverted light, you need to interfere it with another light beam and then use coincidence detection to filter out the desired pattern. Filtering out the light going off at angles will not help in this regard, as it will not generate an interference pattern.