Short Light.

The world’s shortest single photon has been produced by physicists at Oxford University. Light can be thought of as a series of waves or, in the dualistic view of reality prescribed by quantum science, as a collection of quanta, particle-like parcels of light energy referred to as photons. At any place along a light beam there may be many photons present or in special cases just one. Creating single photons is not easy to do. It is possible to make photons in pairs by sending laser light through special crystals. Even a pure-color laser beam will consist of many photons; but occasionally one of these photons will be “down converted,” that is, will turn into two photons each with half the energy of the original photon. When a pair has been created, the detection of one of these half-energy photons heralds the presence of its twin.

Furthermore, these photons are entangled, meaning that the properties of one photon are inextricably linked to those of its partner and detecting one can ruin the quantum state of the other. By minimizing these quantum correlations, the researchers obtained heralded photons with exceptionally high quality and short duration. In the Oxford experiment the pairs of photons made had a central wavelength of about 830 nm, at the border between visible and near-infrared light. Each of these photons was (in units of time) about 65 femtoseconds (65 x 10^-15 sec) long. In units of space, they were about 20 microns long.

The shortest previously produced single photon was about 1picosecond (10^-12 sec) long. Even shorter pulses of light-stretching only hundreds of attoseconds-have been made, but these pulses consist of many photons. One of the Oxford researchers, Peter Mosley (p.mosely1@physics.ox.ac.uk, 44-01865-282640), says that this new experiment represents the first time that textbook photons-identical, localized wavepackets containing a single quantum of energy-have been produced in a lab. (Mosley et al., Physical Review Letters upcoming article)