First light for one-atom laser...

http://physicsweb.org/article/news/7/9/10

First light for one-atom laser
17 September 2003

Physicists in the US have built a laser with a single atom for the first
time. Jeff Kimble and colleagues at the California Institute of Technology
made the device by trapping a cold caesium atom in an optical cavity. The
one-atom laser produces nonclassical light that could have applications in
quantum information technology (J McKeever et al. 2003 Nature 425 268).

In a conventional laser an optically active material is placed between two
mirrors in a cavity and then excited by an electrical current or another
laser. The photons emitted by the optically active material are reflected
back through the material, stimulating the emission of further photons and
leading to the production of laser radiation. These lasers operate in the
'weak-coupling' regime, which involves large numbers of atoms and photons.
This means that the quantum fluctuations of individual electrons or photons
have little impact on the overall laser system.





The device built by the Caltech team is different in that the atom and the
photons are strongly coupled. Kimble and colleagues start by cooling and
trapping a single caesium atom in a tiny optical resonant cavity. Next a
laser is used to excite the caesium atom, which then decays to an
intermediate state and emits a photon. The strong coupling between the atom
and cavity means that this photon is almost always emitted into a laser
cavity mode rather than free space. Another laser is then used to transfer
the atom to another excited state, from which it decays back to the ground
state - allowing the whole processed to be repeated.

The light emitted by one-atom laser exhibits photon antibunching - a quantum
effect which means that it is "quieter" or more ordered than light from
conventional lasers. It also differs from conventional lasers in that there
is no threshold for lasing. Although the single-atom laser has a very small
flux - less than 100 000 photons per second - and only operates for about a
second, Kimble and co-workers say that their result "presses laser operation
to its conceptual limit".

Author
Belle Dumé is Science Writer at PhysicsWeb
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