Single-Photon Atomic Cooling - Raizen Lab - The University of ...

the momentum kicks due to absorption will sum. The result is that atoms
traveling towards the red detuned beam will be slowed by momentum transfer
from scattered photons. This arrangement does more than simply slow the
center-of-mass motion of an ensemble, it also cools it because the red detuned
beam affects atoms in a velocity dependent way. Quickly moving atoms are
Doppler shifted to the blue more than slowly moving atoms (assuming both
are traveling toward the laser source), causing them to scatter at a greater rate
which in turn decelerates them more quickly than more slowly moving atoms.
This compresses the velocity spread of the ensemble, thereby cooling it.
Experiments applying this cooling scheme to electromagnetically trapped
ions were successfully performed in 1978 [4, 5]. Then in 1985 the first exper-
iments demonstrating slowing of neutral atomic beams were published [6, 7].
That same year, trapping in velocity space with counter propagating red de-
tuned beams, an arrangement known as optical molasses, was demonstrated
at Bell Labs by a group led by S. Chu [8]. The spatial confinement of sodium
atoms using a magnetic field was also demonstrated in Gaithersburg by the
group of W. Phillips [9] in that very productive year of 1985. Two years later,
the combination of ideas utilized in these two experiments were used to both
confine and cool atoms in a technique known as magneto-optical trapping
(MOT) [10].
Shortly after this achievement, groups began to observe and report a
troubling, or perhaps welcome, disagreement between experiment and theory.
In particular, the group of W. Phillips reported temperatures of atomic sam-
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