If single photon interference can form Newton's rings I wonder if anyone knows if any experiment has been done using single-photon interference to form fringe by air wedge, such as Newton's rings. The air wedge could be formed between two flat glass plates or a convex special glass surface and an optically flat plate.
The reason I ask is I don't know if this interference is formed by two photons or just a single photon.
I know single-photon interference can form using double slits.
 A: Single photon interference can certainly form Newton's rings.  You could test this yourself:

*

*Direct a laser beam through a microscope objective to spread out the beam.

*Direct the spread-out beam through the standard optical setup to produce Newton's rings on photographic film or a highly sensitive detector array.

*Attenuate the beam until you can be sure only a single photon is in the setup at any one moment

*Record the light that reaches the film or detector array over a long time

The result will be that you obtain Newton's Rings as the statistical sum of all the individual photon detections by the film or detector array.
I don't know if the experiment as described has been done, but there is no doubt that it will produce Newton's rings because ultimately it's no different from the two-slit experiment.
A: I concur with the assertion that no matter how low the luminosity of the light that reaches the photodetection device it will always be possible to obtain an interference pattern.
One particular striking demonstration of that was in a 1967 experiment by Pfleegor and Mandel: Interference of independent photobeams

The setup used two independent lasers, set up to emit light with the same frequency as close as possible. (Still, laser sources drift in frequency, so the setup had a way of identifying moments when the random frequency drifting brought the two beams very close to each other. At those specific moments measurements were taken.)
The emitted light was attenuated with filters, such that the photomultiplier tube setup that was used registered single photo reception events. (Average time between consecutive events: 2 microseconds)
Any photon reaching the photomultiplier tubes could have originated with either of the two sources. The measurements implied that interference effects were obtained.
More information about this setup is available in a blog post by Chad Orzel, titled The Pfleegor-Mandel Experiment

The outcome of the Pfleegor-Mandel experiment strongly suggest that attempts to account for interference effects in terms of 'a photon interfering with itself' fall short.
Instead the outcome of the Pfleegor-Mandel experiment strongly suggests that in order to account for interference effects properly the setup as a whole must be considered. The proper way to account for interference effects, it appears, entails that for the process of interference the luminosity is immaterial.
