Optical switches are extremely large compared to contemporary transistors, and there's no obvious way to scale them smaller than the diffraction limit.
Hopefully this kind of problem will be solved within a decade, do you think that's a reasonable time scale or is this problem even unsolvable?
Like all research into the unknown it might prove easy (5-10 years) it might prove hard (20 years) it might always be 50 years away (hehe fusion) or it might just not be possible (faster than light travel).