photons as their substrate

Photons can't interact with each other (by the linearity of Maxwell's equations) and so can't form a computational substrate on their own. This doesn't rule out "no atoms" computing in general though.

EDIT: I'm wrong. When you do the calculations in full quantum field theory there is a (extremely) slight interaction (due to creations and destructions of electron-postitron pairs, which in some sense destroy the linearity). I don't know if this is enough to support computers.