Indeed, it is nuanced, as you point out. For example, you can't just attenuate a laser and use that as a single photon source (instead you'd get a coherent state). To realise a true single photon source you need an additional (quantum) process, like controlled stimulated emission from single atoms, or driving some nonlinear crystal to generate photon pairs (that's using spontaneous parametric down conversion, i think). And that's where the coherence properties of the laser are essential.
As for fully integrated optics, it's where quantum computers eventually want to be, and there's no physical limitations currently. But perhaps it's too early to say whether we would absolutely require free space optics because it's impossible to do some optics thing another way.
As for fully integrated optics, it's where quantum computers eventually want to be, and there's no physical limitations currently. But perhaps it's too early to say whether we would absolutely require free space optics because it's impossible to do some optics thing another way.