Magneto-optical trap

A magneto-optical trap (abbreviated MOT) is an apparatus that uses laser cooling with magneto-optical trapping in order to produce samples of cold, trapped, neutral atoms at temperatures as low as several microkelvins, two or three times the recoil limit (see Doppler cooling limit). By combining the small momentum of a single photon with a velocity and spatially dependent absorption cross section and a large number of absorption-spontaneous emission cycles, atoms with initial velocities of hundreds of metres per second can be slowed to tens of centimetres per second.

Although charged particles can be trapped using a Penning trap or a Paul trap using a combination of electric and magnetic fields, those traps do not work for neutral atoms.

Photons have a momentum given by ${\displaystyle \hbar k}$ (where ${\displaystyle \hbar }$ is the reduced Planck constant and ${\displaystyle k}$ the photon wavenumber), which is conserved in all atom-photon interactions. Thus, when an atom absorbs a photon, it is given a momentum kick in the direction of the photon before absorption. By detuning a laser beam to a frequency less than the resonant frequency (also known as red detuning), laser light is only absorbed if the light is frequency up-shifted by the Doppler effect, which occurs whenever the atom is moving towards the laser source. This applies a friction force to the atom whenever it moves towards a laser source.

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