This page is still in progress.Single-Photon Hologram of a Zero-Area Pulse. M. Lipka and M. Parniak in Phys. Rev. Lett. 127:163601 (2021).  What the paper says!?

The Authors study the frequency-resolved (wavelengths) HOM effect, with one photon of a SPDC bi-photon pair passing through a Rydber gas to imprint a non-trivial phase.

The following statement (in contrast to our approach) overlooks that the underlying physics extends beyond single-photons to all quantum states, and that there are non-trivial HOM features even without adding "qualitative features":

The nonclassical features of SPs become intriguing when the spatial or temporal mode has a nontrivial structure with added qualitative features, such as orbital angular momentum (OAM).

They comment on the «highly unintuitive phenomena bringing together the bosonic and quantum nature of SP light» (SP = single-photon)

Their Refs. 13-14 point to HOM with atoms.