Laser cooling

Laser cooling includes several techniques where atoms, molecules, and small mechanical systems are cooled with laser light. The directed energy of lasers is often associated with heating materials, e.g. laser cutting, so it can be counterintuitive that laser cooling often results in sample temperatures approaching absolute zero. It is routinely used in atomic physics experiments where the laser-cooled atoms are manipulated and measured, or in technologies, such as atom-based quantum computing architectures.
Laser cooling reduces the random motion of particles or the random vibrations of mechanical systems. For atoms and molecules this reduces 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.
Laser cooling relies on the momentum change when an object, such as an atom, absorbs and re-emits a photon. Atoms will be cooled in one dimension if they are illuminated by a pair of counter-propagating laser beams whose frequencies are below the atoms' laser-cooling transition. The laser light will be preferentially absorbed from the laser beam that counter-propagates with respect to the atom's motion due to the Doppler effect. The absorbed light is re-emitted by the atom in a random direction. After this process is repeated the random motion of the atoms will be reduced along the laser cooling axis. With three pairs of counter-propagating laser beams along all three axes a warm cloud of atoms will be cooled in three dimensions. The atom cloud will expand more slowly because of the decrease in the cloud's velocity distribution, which corresponds to a lower temperature and therefore colder atoms. For an ensemble of particles, their thermodynamic temperature is proportional to the variance in their velocity, therefore the lower the distribution of velocities, the lower the temperature of the particles.

History

Radiation pressure

is the force that electromagnetic radiation exerts on matter. In 1873, James Clerk Maxwell published his treatise on electromagnetism in which he predicted radiation pressure. The force was experimentally demonstrated for the first time by Pyotr Lebedev and reported at the International Congress of Physics during the 1900 Exposition Universelle in Paris, and later published in more detail in 1901. Following Lebedev's measurements Ernest Fox Nichols and Gordon Ferrie Hull also demonstrated the force of radiation pressure in 1901, with a refined measurement reported in 1903. Svante Arrhenius and Peter Debye identified that the gas tail of a comet pointing away from the Sun is due to radiation pressure.
The state of an atom or molecule can be changed by light when it drives a transition between states. Transitions are strongly driven when the light's frequency is near an atomic or molecular transition frequency. Sodium is historically notable atom because it has a strong transition at 589 nm, a wavelength which is close to the peak sensitivity of the human eye. This made it relatively easy to see the interaction of light with sodium atoms. In 1933, Otto Frisch deflected an atomic beam of sodium atoms with light.
This was the first realization of radiation pressure acting on an atom or molecule.

Laser cooling proposals

The introduction of lasers in atomic physics experiments was the precursor to the laser cooling proposals in the mid 1970s. Laser cooling was proposed separately in 1975 by two different research groups: Theodor W. Hänsch and Arthur Leonard Schawlow, and David J. Wineland and Hans Georg Dehmelt. Both proposals outlined the simplest laser cooling process, known as Doppler cooling, where laser light tuned below an atom's resonant frequency is preferentially absorbed by atoms moving towards the laser and after absorption a photon is emitted in a random direction. This process is repeated many times and in a configuration with counterpropagating laser cooling light the velocity distribution of the atoms is reduced.
In 1977 Arthur Ashkin submitted a paper which describes how Doppler cooling could be used to provide the necessary damping to load atoms into an optical trap. 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. He also discussed overlapping optical traps to study interactions between different atoms.

First laser cooling results

Following the laser cooling proposals, in 1978 two research groups, that of Wineland, Robert Drullinger and Fred Walls of National Institute of Standards and Technology, and that of Werner Neuhauser, Martin Hohenstatt, Peter E. Toschek and Dehmelt of the University of Washington succeeded in laser cooling atoms. The NIST group was motivated laser cool atoms to reduce the effect of Doppler broadening on spectroscopy. They cooled magnesium ions in a Penning trap to below 40 K. The Washington group cooled barium ions.
In Russia, Victor Balykin, and Vladimir Minogin at the Institute for Spectroscopy Russian Academy of Sciences in Moscow, realized the first experiment demonstrating laser cooling of neutral atoms in 1981. Aside from that, Letokhov also developed the frequency chirping to slow down atoms but was unsuccessful at applying it.
Influenced by the Wineland's work on laser cooling ions, William Daniel Phillips laser cooled neutral atoms. In 1982, he published a paper where neutral atoms were laser cooled. The process used is now known as the Zeeman slower and is a standard technique for slowing an atomic beam. In 1985, they improved the experiment using a magneto-optical trap devised by David E. Pritchard.
In 1985, John L. Hall at NIST was able fully stop the atoms using the frequency chirp technique.

Optical molasses and atomic fountains

In 1985, Steven Chu, Ashkin and J. E. Bjorkholm at Bell Labs, attempted to realize the Doppler cooling proposal of Hänsch and Schawlow using a pre-frequency chirped cooled laser, and later the atoms drifted into six pair-wise laser beams. The motion of the atoms was described as a viscous medium that they called optical molasses. They determined that the temperature was about 0.24 mK with a lifetime of 0.1 s. In 1987, Pritchard and Chu developed a better magneto-optical trap suggested by Jean Dalibard, using three counter-propagating lasers, circular polarization and a weak magnetic field.
Suggested by Jerrold R. Zacharias and Hänsch, Chu created an atomic fountain for spectroscopy measurements. Using Norman Ramsey Jr. idea, atoms were resonantly excited by a succession of two microwave pulses. This technique allowed the creation of precise atomic clocks.

Sub-Doppler cooling and sub-recoil cooling

Philips at NIST in 1988 discovered that optical molasses could reach 40 micro-kelvin, far below the temperatures predicted by theoretical the Doppler limit, a phenomenon they dubbed Sisyphus cooling. The violation of the limit was confirmed by Chu at Stanford University and by Claude Cohen-Tannoudji and Dalibard at the École normale supérieure in Paris, who explained the phenomena in terms of polarization gradient cooling. In 1989, Philips collaborated in further experiments in the École normale to explain the phenomenon. This sub-Doppler cooling is limited by the recoil temperature.
In the 1970s, experiments by Ennio Arimondo in the University of Pisa showed that the atomic cloud could be set into a dark state. Cohen-Tannoudji, Arimondo and Alain Aspect used this to develop velocity selective coherent population trapping. This technique was first applied in 1D in 1988, in 2D in 1994, and in 3D in 1995. By using this technique, they were able to go below the recoil limit for 1D in 1988 by a factor of 2. Cohen-Tannoudji and Aspect showed in 1994 the principle in 2D, going below the recoil temperature limit by a factor of 16. And finally in 1995, showed a 3D experiment where they went below the limit by a factor of 21.
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".

Modern advances

Atoms

The Doppler cooling limit for electric dipole transitions is typically in the hundreds of microkelvins. In the 1980s this limit was seen as the lowest achievable temperature. It was a surprise then when sodium atoms were cooled to 43 microkelvins when their Doppler cooling limit is 240 microkelvins, this unforeseen low temperature was explained by considering the interaction of polarized laser light with more atomic states and transitions. Previous conceptions of laser cooling were decided to have been too simplistic. 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. The new state of matter, the Bose–Einstein condensate, was observed in 1995 by Eric Cornell, Carl Wieman, and Wolfgang Ketterle.

Exotic Atoms

Most laser cooling experiments bring the atoms close to at rest in the laboratory frame, but cooling of relativistic atoms has also been achieved, where the effect of cooling manifests as a narrowing of the velocity distribution. In 1990, a group at JGU successfully laser-cooled a beam of ⁷Li⁺ at in a storage ring from to lower than, using two counter-propagating lasers addressing the same transition, but at and, respectively, to compensate for the large Doppler shift.
Laser cooling of antimatter has also been demonstrated, first in 2021 by the ALPHA collaboration on antihydrogen atoms. In 2024, positronium, made up of an electron and a positron, was laser cooled to about 1K.