A SINGLE-PHOTON TURNSTILE, a device in which photons are emitted one at a time under controlled circumstances, has been created by a team of scientists from Stanford (US), Hamamatsu Photonics (Japan), and NTT (Japan). Essentially the researchers use the quantization of electrical conductance to produce a quantization of photon emission. They put together a quantum well (the frontier between two thin semiconductor layers) containing a single electron (other electrons are dissuaded from entering because of a "Coulomb blockade" effect) with a quantum well containing a lone (comparably Coulomb blockaded) hole, and then cycle the voltage across the whole stack of layers in such a way that the lone electron and lone hole meet, mate, and make a lone photon. The resulting device, which operates at mK temperatures, is typically a tiny post some 700 nm tall and with a diameter of 200-1000 nm. (J. Kim et al., Nature, 11 February 1999.)