Zeeman slower
These atoms effuse out of a hole in the oven with average speeds on the order of hundreds of m/s and large velocity distributions (due to their high temperature).
The Zeeman slower is attached close to where the hot atoms exit the oven and are used to slow them to less than 10 m/s (slowing) with a very small velocity spread (cooling).
A Zeeman slower consists of a cylinder, through which an atomic beam travels, a pump laser that counterpropagates with respect to the beam's direction, and a magnetic field (commonly produced by a solenoid-like coil) that points along the cylinder's axis with a spatially varying magnitude.
The spatially varying magnetic field is designed to Zeeman-shift the resonant frequency to match the decreasing Doppler shift as the atoms are slowed to lower velocities while they propagate through the Zeeman slower, allowing the pump laser to be continuously resonant and provide a slowing force.
The Zeeman slower was first developed by Harold J. Metcalf and William D. Phillips (who was awarded 1/3 of the 1997 Nobel Prize in Physics in part work for his work on the Zeeman slower[1]).
[2] The achievement of these low temperatures led the way for the experimental realization of Bose–Einstein condensation, and a Zeeman slower can be part of such an apparatus.
If it moves in a specific direction and encounters a counter-propagating laser beam resonant with its transition, it is very likely to absorb a photon.
The absorption of this photon gives the atom a "kick" in the direction that is consistent with momentum conservation and brings the atom to its excited state.
The photon will be reemitted (and the atom will again increase its speed), but its direction will be random.
When averaging over a large number of these processes applied to one atom, one sees that the absorption process decreases the speed always in the same direction (as the absorbed photon comes from a monodirectional source), whereas the emission process does not lead to any change in the speed of the atom because the emission direction is random.
Thus the atom is being effectively slowed down by the laser beam.
There is nevertheless a problem in this basic scheme because of the Doppler effect.
The resonance of the atom is rather narrow (on the order of a few megahertz), and after having decreased its momentum by a few recoil momenta, it is no longer in resonance with the pump beam because in its frame, the frequency of the laser has shifted.
The Zeeman slower[4] uses the fact that a magnetic field can change the resonance frequency of an atom using the Zeeman effect to tackle this problem.
The average acceleration (due to many photon absorption events over time) of an atom with mass
is the saturation intensity of the laser) is In the rest frame of the atoms with velocity
The most common approach is to require that we have a magnetic field profile that varies in the
direction such that the atoms experience a constant acceleration
It has been recently shown, however, that a different approach yields better results.
(which determines the required laser intensity) is normally chosen to be around 0.5.
, then after absorbing a photon and moving to the excited state, the atom would preferentially re-emit a photon in the direction of the laser beam (due to stimulated emission), which would counteract the slowing process.
Alternative designs include a single-layer coil that varies in the pitch of the winding[6] and an array of permanent magnets in various configurations.
The final speed to be reached is a compromise between the technical difficulty of having a long Zeeman slower and the maximal speed allowed for an efficient loading into the trap.
A limitation of setup can be the transverse heating of the beam.
These fluctuations are linked to the atom having a Brownian motion due to the random reemission of the absorbed photon.
