Optical Transistor Flips On with One Photon

A computer based entirely on optical components could perform some tasks much faster than existing computers, which rely on electronic components whose speed is limited by heat generation. However, a fundamental limitation to making the components for an all-optical processor is that photons do not interact: unlike electrons that interact strongly and therefore can easily be used to manipulate other electrons, interactions between photons are generally too weak for one or several photons to be able to block or redirect others. However, recent experiments have shown that highly excited atomic states could mediate the necessary coupling between photons [1, 2, 3]. Experimentalists have used these states to make a single-photon source [2], a single-photon phase shifter [3], and now, as reported by two independent research groups in Physical Review Letters, a single-photon transistor [4, 5].