Magneto-optical trap
In atomic, molecular, and optical physics, a magneto-optical trap (MOT) is an apparatus which uses laser cooling and a spatially varying magnetic field to create a trap which can produce samples of cold neutral atoms.
Temperatures achieved in a MOT can be as low as several microkelvins, depending on the atomic species, which is two or three times below the photon-recoil limit.
However, for atoms with an unresolved hyperfine structure, such as 7Li, the temperature achieved in a MOT will be higher than the Doppler cooling limit.
A MOT is formed from the intersection of the zero of a weak quadrupolar magnetic field and six circularly polarized red-detuned optical molasses beams.
The polarization of the beam propagating in the opposite direction of this atomic motion is chosen to drive this transition.
The absorption of these photons gives rise to a scattering force that pushes the atoms back towards the center of the trap.
In this way, a MOT is able to trap and cool atoms over repeated absorption and spontaneous emission cycles with initial velocities of hundreds of meters per second down to tens of centimeters per second (again, depending upon the atomic species).
The beams must be circularly polarized to ensure that photon absorption can only occur for certain transitions between the ground state
In other words, the circularly-polarized beams enforce selection rules on the allowed electric dipole transitions between states.
Because this "kick" from the emitted photon occurs in a random direction, the net effect of many absorption-spontaneous emission events will result in the atom being "pushed" back towards the field-zero of the trap.
At the center of the trap, the magnetic field is zero and atoms are "dark" to incident red-detuned photons.
That is, at the center of the trap, the Zeeman shift is zero for all states and so the transition frequency
The detuning of the photons from this frequency means that there will not be an appreciable amount of absorption by atoms in the center of the trap, hence the term "dark".
Thus, the coldest, slowest moving atoms accumulate in the center of the MOT where they scatter very few photons.
Mathematically, the radiation pressure force that atoms experience in a MOT is given by:[1]
Because of this, if an atom is to be laser cooled, it must possess a specific energy level structure known as a closed optical loop, where following an excitation-spontaneous emission event, the atom is always returned to its original state.
state, which would require an angular momentum change of −2, which cannot be supplied by a single photon.
Many atoms that do not contain closed optical loops can still be laser cooled, however, by using repump lasers which re-excite the population back into the optical loop after it has decayed to a state outside of the cooling cycle.
transition is used to recycle the population back into the optical loop so that cooling can continue.
These lasers need stability, rather than high power, requiring no more than the saturation intensity, but a linewidth much less than the Doppler width, usually several megahertz.
Because of their low cost, compact size and ease of use, laser diodes are used for many of the standard MOT species while the linewidth and stability of these lasers is controlled using servo systems, which stabilises the lasers to an atomic frequency reference by using, for example, saturated absorption spectroscopy and the Pound-Drever-Hall technique to generate a locking signal.
By employing a 2-dimensional diffraction grating it is possible to generate the configuration of laser beams required for a magneto-optical trap from a single laser beam and thus have a very compact magneto-optical trap.
[2] The MOT cloud is loaded from a background of thermal vapour, or from an atomic beam, usually slowed down to the capture velocity using a Zeeman slower.
This means that the MOT cloud only forms in a vacuum chamber with a background pressure of less than 100 micropascals (10−9 bar)}.
[3] The minimum temperature and maximum density of a cloud in a magneto-optical trap is limited by the spontaneously emitted photon in cooling each cycle.
The absorption, by a neighboring atom, of a spontaneously emitted photon gives a 2ħk momentum kick between the emitting and absorbing atom which can be seen as a repulsive force, similar to coulomb repulsion, which limits the maximum density of the cloud.
[4][5] Because of the continuous cycle of absorption and spontaneous emission, which causes decoherence, any quantum manipulation experiments must be performed with the MOT beams turned off.
In this case, it is common to stop the expansion of the cloud while the MOT is off by loading the cooled atoms into a dipole trap.
Atoms are cooled in a MOT down to a few times the recoil limit, and then evaporatively cooled which lowers the temperature and increases the density to the required phase space density.
[citation needed] MOTs are used in a number of quantum technologies (i.e. cold atom gravity gradiometers) and have been deployed on several platforms (i.e. UAVs) and in several environments (i.e. down boreholes [6]).
