Credit: © 2007 Nature Physics

Normally, the interaction between two photons — the quanta of light — is so weak that it is difficult to control the path of one photon with another. A proposal for a device which controls the transmission of light with a single photon — a single-photon transistor — gets around this problem by taking advantage of the strong interaction between the collective excitations of electrons (plasmons) confined to the surface of a nanowire and an optical emitter.

The key to the theoretical device proposed by Mikhail Lukin and colleagues at Harvard University in the US and the Niels Bohr Institute in Denmark1 is that the light emitted from surface plasmons can be very tightly confined, even in an annulus that is much smaller than the wavelength of the light. As a result, the plasmons produce a very intense electric field that interacts non-linearly with a nearby optical emitter, such as a quantum dot or molecule. The plasmons can therefore be used to prepare the emitter in a state that allows, or prevents, the passage of light along the nanowire.

From the standpoint of quantum optics, the device is an interesting conceptual proposal and if it is successfully built, the nanowire optical transistor could find an application as a single-photon detector.