Laser physicists are good at producing and manipulating single
photons, but as with good comedy, the timing is important. Even
the best experiments in quantum cryptography and
computing–applications that make use of single photon
properties–use sources that emit photons at random times. In the
4 October PRL a French team demonstrates a system that emits
single photons on a dependable schedule at a frequency of 3 MHz.
One other “triggered” photon source which operates on completely
different principles was reported earlier this year. With these
new techniques, researchers know exactly when and where a single
photon will be found, and they are a step closer to quantum
applications, such as cryptography that allows the receiver of
information to deduce whether a message has been intercepted.
Imagine a photon counter in front of a very weak laser beam. The
number of photons reaching the detector in a given period of
time (the laser power) may be precisely known, but the photons
will arrive at random times. Even a