I worked in quantum optics for a while. Our DARPA grant once had the "mission" to see how many bits of information could be theoretically crammed into 1 photon. It turns out to be an uninteresting question because you can theoretically cram infinite bits into one photon, encoded in the relative timing of the photon in a pulse train, limited only by the dispersion of your medium (in space, effectively zero). Even dispersion is a boring question because it is possible to reverse dispersion by sending the light through a parametric amplifier to conjugate the phases and then running it through the dispersion medium a second time locally.

We later ended up working on other things.