Coincidence and Single Rate detection in Quantum Eraser experiments

[moving_on]

I am having difficulty understanding why the single rate of detection
(as opposed to the coincidence rate) is unaffected during Quantum Eraser experiments.
I'm looking at the great Kwait, Steinberg and Chiao paper:
'Observation of a "quantum eraser": A revival of coherence in a two-photon
interference experiment' Phys. Rev. A V. 45, No. 11 1/6/1992.
Say, for example, p. 7731:
'This demonstrates the coincidence dip at zero path-length difference to the beam
splitter. Note that the singles rate at either detector, given by ..., does not show
this dependence on path-length difference'.
I should probably try harder to get hold of the earlier papers quoted but I fear
I would need a less technical description anyway.
Probably a dumb thing to say, but a 'rate' is obviously the number of detections over
a set period of time. Surely one would expect a difference in such a 'rate' when there
is a greater likelihood of both photons going to one detector than when there is a
greater likelihood of both photons going to different detectors?
Or am I just being really, really stupid...

 

[azntoon]

The single rate of detection is unaffected in a quantum eraser experiment because the presence or absence of which-path information does not change the probability of detecting one photon. The two-photon interference pattern that is observed is due to the wave nature of the photons, meaning that the probability of detecting a single photon is the same regardless of whether you know which path it took or not. The coincidence rate, however, depends on the presence or absence of which-path information, since knowing which path each photon took allows you to determine if they were detected at the same time or not. This is why the coincidence rate changes when the which-path information is available.