what I'm thinking is that you don't really need the light to do work, as such. If you can build a microscopic, say, XOR gate based on light, which accepts two input light streams, and outputs a single stream that is the XOR of the inputs, you can start building a CPU using light. Perhaps there is some material that, when shined with the light from a single source, it emits light, but when shined with twice the light, it blocks the light. That would be an XOR gate, as far as I can see, and it would not be running on a clock -- the two input streams combine inside the gate to produce the output.