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One atom trapped between a
pair of mirrors and
optically pumped from the side: this is the fundamental "conceptual"
lower
limit on the size of a laser. The basic idea is not really to
build a device that will be
used
for any practical application, nor is it an exercise in miniaturization
of lasers. It's more of an exploration of a fundamental limit, in
a particularly quantum & strongly-coupled regime. The output
of
our device is
non-classical, meaning our photodetection data can
only
come from a quantized field. Although the properties of the
system
are not typical for a laser, that is a result of the strong coupling of
the atom to the field (or equivalently, the very small size of our
cavity)
which effectively causes each photon to leave the cavity as soon as it
is "created" by the driven atom (at random moments). |
Single photons on demand.
Using a pulsed sequence of side beams, we are able to generate single photons deterministically from atoms trapped in a cavity.
- About 14000 photons are produced by each atom on average
- 2-photon events are suppressed by over twenty-fold relative
to a weak coherent state. This is limited by rare events in which
we load 2 atoms into the trap.
- Our overall escape, propagation and detection efficiency is between 2 and 3%, in agreement with the fraction of attempts in which we detect a photon.
- Our system enables coherent reversibility and user-controlled pulse shapes, which we are working to improve in ongoing experiments.
The
experiments above rely on an external red-detuned classical field
in order to trap atoms, but it is also possible to use the atom-cavity
interaction itself. By making the coupling
energy of the atom to the cavity, g, greater than kinetic
energy of the atom, we were able to trap
single atoms with a single-photon-strength laser field for the first
time. Quicktime movies of individual atom trajectories from this atom-cavity
microscope are available here.
One exciting prospect is that by observing atoms passing through the
standing wave structure of the cavity mode, it should be possible to
approach the standard quantum limit for measurement of the atomic
position. We have designed and analyzed the performance of feedback
algorithms which would actively stablilize the motion of the atom
in real time.