laser cooling

n.
The slowing down of atoms or molecules by the use of a laser whose frequency has been adjusted to remove momentum from the particles.

Reducing the thermal motion of atoms with the force exerted by a laser beam. Typically, such cooling is used to reduce the temperature of a gas of atoms, or the velocity spread of atoms in an atomic beam.

Light affects atomic motion when the atoms absorb or emit photons, the particles or quanta that make up light. Photons carry momentum p = h/λ, where h is Planck's constant and λ is the light's wavelength. By conservation of momentum, when an atom absorbs or emits a photon, the atom's momentum must change by an amount equal to the photon momentum. Each absorption or emission thus gives the atom a tiny kick, changing its velocity. For most atoms this change is only a few millimeters to a few centimeters per second, while atoms in a gas at room temperature have velocities of a few hundred to a few thousand meters per second. Nevertheless, repeated absorption and emission of photons can have a significant effect on even hot atomic gases or beams. See also Conservation of momentum; Light; Momentum; Photon.

The keys to using such repeated kicks to reduce the random, thermal motion of a gas of atoms are the monochromatic nature of laser light, the selectivity of absorption of light by atoms, and the Doppler effect. Light is an oscillating electromagnetic wave whose frequency of oscillation determines its color. The energy of each photon is E = hν, where ν is the frequency. Laser light can have nearly a single frequency or color, so that all the photons have almost identical energies. Atoms absorb only photons whose energy is equal, within a small range, to the difference in energy between two of its quantum states or energy levels. For sodium atoms this resonance frequency is ν₀ ≡ 5 × 10¹⁴ Hz (wavelength λ ≡ 589 nanometers), but the absorption is efficient only over a range Δν = 10⁷ Hz. Moving atoms, however, experience a Doppler shift so that, depending on their speed and whether they are moving along the direction of the laser beam or against it, the light appears to room-temperature atoms to have a frequency shifted up or down by a hundred or more times the natural absorption width Δν. See also Doppler effect; Laser.

If the frequency ν of the laser is tuned to be slightly lower than ν₀, those atoms moving against the laser beam see the laser upshifted, closer to ν₀. These atoms are more likely to absorb photons, receive kicks opposite to the direction of their velocity, and slow down. After absorbing a photon, the atoms are in an excited state and return to the original state by spontaneously emitting a photon. Such photons are radiated in random directions, so the effect of their kicks averages to zero. For atoms held in a trap, as ions generally are, any trapped atom will at some time be traveling against the laser beam and be cooled. Laser cooling was first demonstrated in 1978 with such trapped ions. For free atoms, another, similarly tuned laser beam is added, aimed in the opposite sense, to cool those atoms moving in the opposite direction. More generally, one uses three pairs of mutually perpendicular, counterpropagating laser beams, all tuned below ν₀. Then, no matter the direction of an atom's velocity, there are one or more laser beams that oppose the velocity and slow the atom.

Improving atomic clocks, where the thermal motion of atoms reduces the precision and accuracy, was a major motivation to developing laser cooling. Laser cooling is also used in atom optics, where well-collimated, monoenergetic atomic beams are more easily and effectively manipulated. In addition, laser cooling has been used to study collisions between very slow atoms. See also Atomic clock; Scattering experiments (atoms and molecules).

Laser cooling is intimately connected with trapping of atoms, because atoms must often be slowed down before they can be held in a trap and because atoms must often be trapped in order to observe laser cooling or its effects. Such effects include cold, trapped ions arranging themselves into a crystal because of the electric repulsion between the charged ions. Neutral atoms can become arrayed on an optical lattice of tiny traps formed by interference between the laser beams used to cool them. In both cases, the spacing between atoms is thousands of times larger than the spacing in solid crystals. Another effect is Bose-Einstein condensation, wherein a gas of atoms whose de Broglie wavelength is comparable to the spacing between atoms has a transition to a state where a significant fraction of the atoms are in the lowest kinetic energy state possible. See also Bose-Einstein condensation; Particle trap; Quantum mechanics.

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