Optical molasses is a

laser cooling Laser cooling includes a number of techniques in which atoms, molecules, and small mechanical systems are cooled, often approaching temperatures near absolute zero. Laser cooling techniques rely on the fact that when an object (usually an atom) ...

technique that can cool neutral

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...

s to temperatures lower than a

magneto-optical trap A magneto-optical trap (MOT) is an apparatus which uses laser cooling and a spatially-varying magnetic field to create a trap which can produce samples of cold, trapped, neutral atoms. Temperatures achieved in a MOT can be as low as several microk ...

(MOT). An optical molasses consists of 3 pairs of counter-propagating

circularly polarized In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electromagnetic field of the wave has a constant magnitude and is rotating at a constant rate in a plane perpendicular to t ...

laser beams intersecting in the region where the atoms are present. The main difference between optical molasses and a MOT is the absence of magnetic field in the former. Therefore, unlike a MOT, an optical molasses provides only cooling and no trapping. While a typical Sodium MOT can cool atoms down to 300μK, optical molasses can cool the atoms down to 40μK, an order of magnitude colder.

History

When laser cooling was proposed in 1975, a theoretical limit on the lowest possible temperature was predicted. Known as the

Doppler limit Doppler cooling is a mechanism that can be used to trap and slow the motion of atoms to cool a substance. The term is sometimes used synonymously with laser cooling, though laser cooling includes other techniques. History Doppler cooling was si ...

T_d=  \hbar \Gamma /

, this was given by the lowest possible temperature attainable considering the cooling of two-level atoms by Doppler cooling and the heating of atoms due to momentum diffusion from the scattering of laser photons. Here,

\Gamma

, is the natural line-width of the atomic transition,

\hbar

, is the

reduced Planck constant The Planck constant, or Planck's constant, is a fundamental physical constant of foundational importance in quantum mechanics. The constant gives the relationship between the energy of a photon and its frequency, and by the mass-energy equivale ...

and,

k_b

, is the

Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, ...

. Experiments at the

National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...

, Gaithersburg, found the temperature of cooled atoms to be well below the theoretical limit. Initially, it was a surprise to theorists, until the full explanation came out.

Theory

The best explanation of the phenomenon of optical molasses is based on the principle of

polarization gradient cooling Polarization gradient cooling (PG cooling) is a technique in laser cooling of atoms. It was proposed to explain the experimental observation of cooling below the doppler limit. Shortly after the theory was introduced experiments were performed tha ...

. Counterpropagating beams of circularly polarized light cause a standing wave, where the

light polarization Polarization ( also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of t ...

is linear but the direction rotates along the direction of the beams at a very fast rate. Atoms moving in the spatially varying linear polarisation have a higher probability density of being in a state that is more susceptible to absorption of light from the beam coming head-on, rather than the beam from behind. This results in a velocity dependent damping force that is able to reduce the velocity of a cloud of atoms to near the recoil limit.

References

{{reflist Atomic, molecular, and optical physics