Classically speaking, light is a wave. In quantum mechanics, however, it is both a wave and particles. Single photons, unlike the masses that pour out of a laser or most other light sources, exhibit discrete, or quantized, properties. So it's not surprising that a source of single photons would be useful in the emerging world of quantum computing and cryptography. The problem has been how to get one photon at a time. Existing methods require extremely low temperatures, around 2 K. Brahim Lounis of Bordeaux University and W.E. Moerner of Stanford University in California, however, have developed a room-temperature technique that produces single photons on demand. The breakthrough is the use of terrylene molecules in p-terphenyl crystals. The researchers, who reported their achievement in the Sept. 28, 2000, issue of Nature, embedded terrylene, which fluoresces at approximately 579 nm, in low concentrations in the crystal. As a result, the terrylene molecules are protected from fluorescent quenchers such as oxygen, and the surrounding crystal helps insulate them from heat damage. In their experiment, Lounis and Moerner pumped the terrylene molecules into an excited state with a 532-nm frequency-doubled Nd:YAG laser. The pulse width was 35 ps, which is much shorter than the 3.8-ns fluorescence decay time of terrylene, and the repetition rate was 160 ns, much longer than the decay time. Thus, each molecule was excited to emit only one photon. "The pulsed excitation allows [a] high rate of single photons with a near-zero probability of two-photon emission," Lounis said. "This is a key point for a secure quantum cryptography.