Sub-Doppler cooling is a class of laser cooling techniques that reduce the temperature of atoms and molecules below the
Doppler cooling 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 ...
.
Doppler cooling
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 ...
processes have a cooling limit that is characterized by the momentum recoil from the emission of a photon from the particle.
Some methods of sub-Doppler cooling include
optical molasses
Optical molasses is a laser cooling technique that can cool neutral atoms to temperatures lower than a magneto-optical trap (MOT). An optical molasses consists of 3 pairs of counter-propagating circularly polarized laser beams intersecting in the ...
,
Sisyphus cooling
In ultra- low-temperature physics, Sisyphus cooling, the Sisyphus effect, or polarization gradient cooling involves the use of specially selected laser light, hitting atoms from various angles to both cool and trap them in a potential well, effec ...
,
evaporative cooling
An evaporative cooler (also known as evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning sy ...
,
free space Raman cooling,
Raman side-band cooling,
resolved sideband cooling Resolved sideband cooling is a laser cooling technique allowing cooling of tightly bound atoms and ions beyond the Doppler cooling limit, potentially to their motional ground state. Aside from the curiosity of having a particle at zero point energy, ...
,
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 ...
, and the use of a dark magneto-optical trap.
For example, an optical molasses time-of-flight technique was used to cool sodium (Doppler limit
) to
.
Some possible motivations for sub-doppler cooling include cooling to the motional ground state, a requirement for maintaining fidelity during many
quantum computation
Quantum computing is a type of computation whose operations can harness the phenomena of quantum mechanics, such as superposition, interference, and entanglement. Devices that perform quantum computations are known as quantum computers. Though ...
operations.
Dark magneto-optical trap
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) is commonly used for cooling and trapping a substance by Doppler cooling. In the process of Doppler cooling, the red detuned light would be absorbed by atoms from one certain direction and re-emitted in a random direction. The electrons of the atoms would decay to an alternative ground states if the atoms have more than one hyperfine ground level. There is the case of all the atoms in the other ground states rather than the ground states of Doppler cooling, then system cannot cool the atoms further.
In order to solve this problem, the other re-pumping light would be incident on the system to repopulate the atoms to restart the Doppler cooling process. This would induce higher amounts of fluorescence being emitted from the atoms which can be absorbed by other atoms, acting as a repulsive force. Due to this problem, the Doppler limit would increase and is easy to meet. When there is a dark spot or lines on the shape of the re-pumping light, the atoms in the middle of the atomic gas would not be excited by the re-pumping light which can decrease the repulsion force from the previous cases.
This can help to cool the atoms to a lower temperature than the typical Doppler cooling limit. This is called a dark magneto-optical trap (DMOT).
[Shengwang Du, Shanchao Zhang, Shuyu Zhou, Guang Yu Yin, and Chinmay Belthangady, "Two-dimensional magneto-optical trap for neutral atoms," US Patent No.: US 8,835,833 B2 (2014); China Patent Pub. No.: CN 102969038 A (2013).]
References
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Atomic, molecular, and optical physics