I have a very fundamental question about interference and quarter-wave-plates. My setup contains only few elements: lightsource, QWPs, double slit and some detectors to see the interference pattern. (Detectors are missing from the bottom of the figure.) An important thing is that the two QWPs are opposite so if a photon comes into QWP1 with polarization x then the QWP1 will make it left circular and if a photon comes into QWP2 with the same polarization x then the QWP2 will make it right circular. My questions is that which of the following cases will one see an interference pattern: 1) source emits normal (not a laser) light 2) source emits laser beam 3) source emits single photons with uncertain polarization (many times) 4) source emits single photons with the same certain known polarization (many times) Thanks in advance for any help.