Ultracold atom

Ultracold atoms are atoms that are maintained at temperatures close to 0 kelvin (absolute zero), typically below temperatures of some tenths of microkelvins (µK). At these temperatures the atom's quantum-mechanical properties become important.

To reach such low temperatures, a combination of several techniques has to be used. First atoms are usually trapped and pre-cooled via laser cooling in a magneto-optical trap. To reach the lowest possible temperature, further cooling is performed using evaporative cooling in a magnetic or optical trap.

Experiments with ultracold atoms are important for understanding quantum phase transition and studying Bose–Einstein condensation (BEC), bosonic superfluidity, quantum magnetism, many-body spin dynamics, Efimov states, Bardeen-Cooper-Schrieffer (BCS) superfluidity and the BEC-BCS crossover.

Samples of ultracold atoms are typically prepared through the interactions of a diffuse gas with a laser field. Evidence for radiation pressure, force due to light on atoms, was demonstrated independently by Lebedev, and Nichols and Hull in 1901. In 1933, Otto Frisch demonstrated the deflection of individual sodium particles by light generated from a sodium lamp.

The invention of the laser spurred the development of additional techniques to manipulate atoms with light. Using laser light to cool atoms was first proposed in 1975 by taking advantage of the Doppler effect to make the radiation force on an atom dependent on its velocity, a technique known as Doppler cooling. Similar ideas were also proposed to cool samples of trapped ions. Applying Doppler cooling in three dimensions will slow atoms to velocities that are typically a few cm/s and produce what is known as an optical molasses.

Typically, the source of neutral atoms for these experiments were thermal ovens which produced atoms at temperatures of a few hundred kelvin. The atoms from these oven sources are moving at hundred of meters per second. One of the major technical challenges in Doppler cooling was increasing the amount of time an atom can interact with the laser light. This challenge was overcome by the introduction of a Zeeman Slower. A Zeeman Slower uses a spatially varying magnetic field to maintain the relative energy spacing of the atomic transitions involved in Doppler cooling. This increases the amount of time the atom spends interacting with the laser light.

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