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Magneto-optical trap
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<p>In <a href="/facts/Atomic,_molecular,_and_optical_physics/6HeJIr7P">atomic, molecular, and optical physics</a>, a magneto-optical trap (MOT) is an apparatus which uses <a href="/facts/Laser_cooling/hU2c6tdk">laser cooling</a> and a spatially varying <a href="/facts/Magnetic_field/Ta8Ev588">magnetic field</a> to create a <a href="/facts/Magnetic_trap_(atoms)/rrx8gGLH">trap</a> which can produce samples of <a href="/facts/Ultracold_atom/LdM0IeD7">cold</a> neutral <a href="/facts/Atom/FoQ4kGN5">atoms</a>. <a href="/facts/Temperature/QyGe4gmj">Temperatures</a> achieved in a MOT can be as low as several <a href="/facts/Microkelvin/ADXGAk8e">microkelvins</a>, depending on the atomic species, which is two or three times below the <a href="/facts/Recoil_temperature/7sIVj0uX">photon-recoil limit</a>. However, for atoms with an unresolved <a href="/facts/Hyperfine_structure/aqj0dMut">hyperfine structure</a>, such as <a href="/facts/Isotopes_of_lithium/XC311sQu">7Li</a>, the temperature achieved in a MOT will be higher than the <a href="/facts/Doppler_cooling/oY5WGWFJ">Doppler cooling</a> limit. </p><p>A MOT is formed from the intersection of the zero of a weak <a href="/facts/Quadrupole_magnet/x2VmpLL8">quadrupolar</a> magnetic field and six <a href="/facts/Circular_polarization/fYQvT0mI">circularly polarized</a> <a href="/facts/Laser_detuning/PFyBxSy5">red-detuned</a> <a href="/facts/Optical_molasses/wJxE2HL2">optical molasses</a> beams. Counterpropagating beams have opposite handed polarization. As atoms travel away from the zero field at the center of the trap, the spatially varying <a href="/facts/Zeeman_effect/A6NMlp2D">Zeeman shift</a> brings an atomic transition into <a href="/facts/Resonance/q7VLALUz">resonance</a> with the laser 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. </p><p>In this way, a MOT is able to trap and cool atoms over repeated absorption and <a href="/facts/Spontaneous_emission/kWWfqQ2V">spontaneous emission</a> cycles with initial velocities of hundreds of meters per second down to tens of centimeters per second (again, depending upon the atomic species). </p>