For over forty years—ever since the invention of the laser in 1960—researchers have sought avidly for a way to compute with light. Why have they done so, and why are computers still electronic, not optical?

In conventional, electronic computing, the elementary carrier of information is the electron. Most computational actions--transmitting signals from one chip to another, storing a bit of information in random access memory, changing the state of a memory circuit, and so on--involve the shifting of electrons in space by the application of electric fields. An electron is charged, so an electric field causes it to accelerate; nearby atoms tend to impede this motion.

In optical computing, the elementary carrier of information is the photon. Depending on the physical context, light can behave either as very small particles (photons) or as a self-propelled wave comprised of an electric field and a...