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)