I've never heard of LightOn, and wish the website had a bit more concrete info on the specifics of the coprocessor, but I am somewhat familiar with a similar photonic coprocessor made by NTT (the Coherent Ising Machine). It's still in the research stage, the logic uses interferometry effects, and requires kilometers of fiber optic cables. Interestingly, there is a simulator based on mean field theory that runs on GPUs and FPGAs(*) that can solve some problems (e.g. SAT) with close to state of the art performance.

(*) disclosure: my company helped build the simulator