One of the most important applications of the single photon source is
quantum cryptography. Up to now, most of the experiments of quantum
cryptography have used weak coherent light as photon sources. In those
experiments, the average photon number was kept as small as 0.1 in
order to maintain a very low probability of generating two photons in
one pulse. However, this meant that 90% of the pulses were vacuum
states and could not be used to send any information. Therefore,
single photon sources that can output single photon states with a high
probability while having only a small probability of two photon states
are very important for quantum cryptography.

There has been extensive research to produce single photon pulses
using a single light emitter, like single molecules, single color
centers in diamonds and single quantum dots. In those methods,
however, photons are emitted in all directions and it is difficult to
collect the photons with high efficiency. To the best of our knowledge,
the highest probability P(1) of finding a single photon at an output
port where single photons are collimated was less than 10% in such
devices.

We report the construction of a single photon source using gated
parametric fluorescence with the measurement result of the photon
number distribution. We succeeded in increasing the probability P(1)
of finding a single photon in a pulse to 26.5%. The availability of
the source at room temperature, its long life, and its wavelength
tunability may also be useful for quantum cryptography and photonic
quantum computation.

For more information,
http://www.nanonet.go.jp/english/mailmag/2004/025b.html