Doppler and Sub-Doppler Cooling

Doppler cooling slows atoms using the velocity-dependent radiation pressure of detuned laser beams, while sub-Doppler mechanisms exploit internal-state structure to reach even lower temperatures.

Definition

Doppler cooling is laser cooling in which the Doppler shift makes an atom scatter more photons from a beam opposing its motion than from one aiding it, producing a velocity-damping force; sub-Doppler cooling refers to mechanisms, such as polarization-gradient cooling, that reach temperatures below the Doppler limit by using the atom's internal sublevel structure.

Scope

This topic covers the principal laser-cooling mechanisms for free atoms: Doppler cooling in counter-propagating red-detuned beams, the optical-molasses configuration, the Doppler cooling limit set by photon recoil, and the sub-Doppler mechanisms—chiefly polarization-gradient (Sisyphus) cooling—that exploit multiple ground sublevels and optical pumping to cool below that limit toward the recoil limit.

Core questions

• How does red detuning of the lasers produce a velocity-dependent cooling force?
• What is optical molasses, and what is the Doppler cooling limit?
• Why do real experiments reach temperatures below the Doppler limit?
• How does polarization-gradient (Sisyphus) cooling work?

Key concepts

• Radiation-pressure (scattering) force
• Red detuning and the Doppler shift
• Optical molasses
• Doppler cooling limit
• Polarization-gradient (Sisyphus) cooling
• Recoil limit

Key theories

Doppler cooling and optical molasses

In three pairs of counter-propagating red-detuned beams, a moving atom sees the opposing beam Doppler-shifted toward resonance and scatters more of its photons, giving a viscous damping force; the residual heating from photon recoil sets the Doppler limit temperature.

Polarization-gradient (Sisyphus) cooling

In a spatially varying light polarization, an atom is repeatedly optically pumped to a sublevel of lower energy after climbing a potential hill, losing kinetic energy each cycle and cooling well below the Doppler limit, as explained by Dalibard and Cohen-Tannoudji.

Clinical relevance

Doppler and sub-Doppler cooling are the first stages in producing the cold atomic samples used in optical atomic clocks, atom interferometers, and quantum technologies, and the discovery that real temperatures fell below the predicted Doppler limit directly motivated the theory of sub-Doppler cooling.

History

Proposed by Hänsch and Schawlow in 1975 and demonstrated as optical molasses by Chu's group in 1985, Doppler cooling was expected to reach a limit of a few hundred microkelvin. When Phillips's group measured temperatures well below this in 1988, Dalibard and Cohen-Tannoudji explained the surprise in 1989 through polarization-gradient cooling.

Key figures

• Theodor Hänsch
• Arthur Schawlow
• Claude Cohen-Tannoudji
• Jean Dalibard

Seminal works

• hansch1975
• dalibard1989

Frequently asked questions

What is the Doppler cooling limit?

It is the lowest temperature Doppler cooling alone can reach, set by the balance between cooling and the heating from the random recoil of spontaneously emitted photons. For typical atomic transitions it corresponds to a few hundred microkelvin.

Why is it called Sisyphus cooling?

In polarization-gradient cooling an atom repeatedly climbs a potential hill, losing kinetic energy, and is then optically pumped back to the bottom of another hill—forever climbing, like the mythological Sisyphus—so it continually sheds energy and cools.