I don't Hong-Ou-Mandel is a two-photon interference. So far in this game there is only a single photon, so you get https://en.wikipedia.org/wiki/Mach%E2%80%93Ze

I don't Hong-Ou-Mandel is a two-photon interference. So far in this game there is only a single photon, so you get https://en.wikipedia.org/wiki/Mach%E2%80%93Zehnder_interferometer https://en.wikipedia.org/wiki/Mach%E2%80%93Zehnder_interfero... and https://simple.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment https://simple.wikipedia.org/wiki/Michelson%E2%80%93Morley_e... interferometers.