laser cooling


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Laser cooling

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 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 = h&ngr;, where &ngr; 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 &ngr;0 ≡ 5 × 1014 Hz (wavelength λ ≡ 589 nanometers), but the absorption is efficient only over a range Δ&ngr; = 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 Δ&ngr;. See Doppler effect, Laser

If the frequency &ngr; of the laser is tuned to be slightly lower than &ngr;0, those atoms moving against the laser beam see the laser upshifted, closer to &ngr;0. 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 &ngr;0. 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 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 Bose-Einstein condensation, Particle trap, Quantum mechanics

laser cooling

[′lā·zər ‚kül·iŋ]
(atomic physics)
A method of slowing atoms in an atomic beam to very low velocities, by directing a beam of properly tuned laser light opposite to the atomic beam, and compensating for the Doppler shift of the slowing atoms by varying the laser frequency or Zeeman-shifting the atomic levels with a varying magnetic field.
References in periodicals archive ?
The quest for ever colder temperatures has been a major theme of physics for more than a century, leading to such breakthroughs as the discovery of superfluidity and superconductivity, and more recently the development of laser cooling techniques.
Gillette et al., "Design and construction of cost-effective tapered amplifier systems for laser cooling and trapping experiments," American Journal of Physics, vol.
A feature of the building is that it is geothermally heated and cooled, with the laser cooling water circulation system incorporated into the ground-based heating/cooling system.
System 25 provides up to 5A laser drive current, precision voltage measurement, two channels of photodetection, selectable photodiode detector types, optional laser diode mount and up to two temperature controllers for laser cooling, all in one kit that can be modified as test needs change.
Key words: laser cooling; photoassociation speetroscopy: scattering length; spectral line shapes; ultracold sodium atom collisions.
Laser cooling and other atom-cooling methods devised in the past 15 years don't work on neutrons since the particles only weakly interact with other matter and light.
The electronic assembly contains the laser drive and laser cooling control, photodetector array drive electronics, a 12-bit A/D converter, digital signal processing board display, PC/AT and an IEEE 488 bus interface.
Their new method for laser cooling, described in the online edition of the journal Nature, is a significant step toward the ultimate goal of using individual molecules as information bits in quantum computing.Currently, scientists use either individual atoms or "artificial atoms" as qubits, or quantum bits, in their efforts to develop quantum processors.
But he added that the ability to apply the technique to a range of molecules makes it exciting: "If laser cooling works, it potentially allows many different molecules to be put into the ultra cold regime."
It links the central topics of optics that were established some 200 years ago to recent research topics such as nonlinear optics, laser cooling, and photonic materials.
Ashkin was selected based on his theoretical and experimental contributions to the understanding of laser cooling and trapping of atoms and particles; for demonstrating the optical gradient forces on atoms and the trapping of atoms with light; and for inventing optical tweezers and showing how they can be used to measure physical forces.

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