Doppler cooling

Doppler cooling was simultaneously proposed by two groups in 1975, the first being David J. Wineland and Hans Georg Dehmelt[1] and the second being Theodor W. Hänsch and Arthur Leonard Schawlow.

[2] It was first demonstrated by Wineland, Drullinger, and Walls in 1978[3] and shortly afterwards by Neuhauser, Hohenstatt, Toschek and Dehmelt.

Steven Chu, Claude Cohen-Tannoudji and William D. Phillips were awarded the 1997 Nobel Prize in Physics for their work in laser cooling and atom trapping.

[5] Doppler cooling involves light with frequency tuned slightly below an electronic transition in an atom.

If the absorption and emission are repeated many times, the mean velocity, and therefore the kinetic energy of the atom, will be reduced.

A few photons happen to "resonate" with the atom in a few very narrow bands of frequencies (a single color rather than a mixture like white light).

The popular idea that lasers increase the thermal energy of matter is not the case when examining individual atoms.

A short time later, the atom will spontaneously emit a photon in a random direction as it relaxes to a lower electronic state.

If the laser were to be positioned so that it was moving towards the observed atoms, then the Doppler effect would raise its frequency.

At one specific velocity, the frequency would be precisely correct for said atoms to begin absorbing photons.

Something very similar happens in a laser cooling apparatus, except such devices start with a warm cloud of atoms moving in numerous directions at variable velocity.

If the atom releases the photon directly to the right, then the dot is redrawn that same distance to the left, putting it back in the narrow band of interaction.

Such an apparatus would be constructed with many lasers, corresponding to many boundary lines that completely surround that cloud of dots.

As the laser frequency is increased, the boundary contracts, pushing all the dots on that graph towards zero velocity, the given definition of "cold".

When a photon is absorbed by an atom counter-propagating to the light source, its velocity is decreased by momentum conservation.

(measured in radians per second), this sets the lower limit to the temperature of the atoms after cooling to be[8]

Mechanisms such as Sisyphus cooling due to multiple ground states lead to temperatures lower than the Doppler limit.

The concentration must be minimal to prevent the absorption of the photons into the gas in the form of heat.

[13] Counter-propagating sets of laser beams in all three Cartesian dimensions may be used to cool the three motional degrees of freedom of the atom.

Common laser-cooling configurations include optical molasses, the magneto-optical trap, and the Zeeman slower.