Laser cooling

Laser cooling relies on the change in momentum when an object, such as an atom, absorbs and re-emits a photon (a particle of light).

The primary benefit of laser cooling is reducing the random motion of particles or mechanical systems.

For atoms and molecules this reduces the effect of Doppler shifts in spectroscopy, allowing for high precision measurements and instruments such as optical clocks.

The reduction in thermal energy also allows for efficient loading of atoms and molecules into traps where they can be used in experiments or atom-based devices for longer periods of time.

Sodium is historically notable because it has a strong transition at 589 nm, a wavelength which is close to the peak sensitivity of the human eye.

Laser cooling was proposed separately in 1975 by two different research groups: Hänsch and Schawlow,[8] and Wineland and Dehmelt.

This process is repeated many times and in a configuration with counterpropagating laser cooling light the velocity distribution of the atoms is reduced.

[10] In 1977 Ashkin submitted a paper which describes how Doppler cooling could be used to provide the necessary damping to load atoms into an optical trap.

[11] In this work he emphasized how this could allow for long spectroscopic measurements which would increase precision because the atoms would be held in place.

The 1997 Nobel Prize in Physics was awarded to Claude Cohen-Tannoudji, Steven Chu, and William Daniel Phillips "for development of methods to cool and trap atoms with laser light".

[15] The major laser cooling breakthroughs in the 70s and 80s led to several improvements to preexisting technology and new discoveries with temperatures just above absolute zero.

The cooling processes were utilized to make atomic clocks more accurate and to improve spectroscopic measurements, and led to the observation of a new state of matter at ultracold temperatures.

Laser cooling reduces the random vibrations of the mechanical oscillator, removing thermal phonons from the system.

In Doppler cooling, initially, the frequency of light is tuned slightly below an electronic transition in the atom.

If the atom, which is now in the excited state, then emits a photon spontaneously, it will be kicked by the same amount of momentum, but in a random direction.

Since the initial momentum change is a pure loss (opposing the direction of motion), while the subsequent change is random, the probable result of the absorption and emission process is to reduce the momentum of the atom, and therefore its speed—provided its initial speed was larger than the recoil speed from scattering a single photon.

For example, gray molasses is used with lithium and potassium because they have unresolved hyperfine structure in their excited states where polarization gradient cooling would not work.

Spectroscopic measurements of a cold atomic sample will also have reduced systematic uncertainties due to thermal motion.

Often multiple laser cooling techniques are used in a single experiment to prepare a cold sample of atoms, which is then subsequently manipulated and measured.

Rubidium, for example is a very commonly used atom which requires driving two transitions with laser light at 780 nm that are separated by a few GHz.

Trapped ions on the other hand require microwatts of optical power, as they are generally tightly confined and the laser light can be focused to a small spot size.