Laser cooling includes a number of 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. 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). For example, if laser light illuminates a warm cloud of atoms from all directions and the laser's frequency is tuned below an atomic resonance, the atoms will be cooled. This common type of laser cooling relies on the Doppler effect where individual atoms will preferentially absorb laser light from the direction opposite to the atom's motion. The absorbed light is re-emitted by the atom in a random direction. After repeated emission and absorption of light the net effect on the cloud of atoms is that they will expand more slowly. The slower expansion reflects a decrease in the velocity distribution of the atoms, which corresponds to a lower temperature and therefore the atoms have been cooled. For an ensemble of particles, their thermodynamic temperature is proportional to the variance in their velocity. More homogeneous velocities between particles corresponds to a lower temperature. Laser cooling techniques combine atomic spectroscopy with the aforementioned mechanical effect of light to compress the velocity distribution of an ensemble of particles, thereby cooling the particles.

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".^[1]

History

Radiation pressure

Radiation pressure is the force that electromagnetic radiation exerts on matter. In 1873 Maxwell published his treatise on electromagnetism in which he predicted radiation pressure.^[2] The force was experimentally demonstrated for the first time by Lebedev and reported at a conference in Paris in 1900,^[3] and later published in more detail in 1901.^[4] Following Lebedev's measurements Nichols and Hull also demonstrated the force of radiation pressure in 1901,^[5] with a refined measurement reported in 1903.^{[6] [7]}

In 1933, Otto Frisch deflected an atomic beam of sodium atoms with light. ^[8] This was the first realization of radiation pressure acting on a resonant absorber.

Laser cooling proposals

The introduction of lasers in atomic manipulation 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: Hänsch and Schawlow, ^[9] and Wineland and Dehmelt.^[10] Both proposals outlined the simplest laser cooling process, known as Doppler cooling, where laser light below an atom's resonant frequency is repeatedly absorbed and the velocity distribution of the atoms is reduced.^[11]

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.^[12] In this work he emphasized how this process could allow for long spectroscopic measurements without the atoms escaping the trap and proposed the overlapping of optical traps in order to study interactions between different atoms.

Initial realizations

Following the laser cooling proposals, in 1978 two research groups that Wineland, Drullinger and Walls of NIST, and Neuhauser, Hohenstatt, Toscheck and Dehmelt of the University of Washington succeeded in laser cooling atoms. The NIST group wanted to reduce the effect of Doppler broadening on spectroscopy. They cooled magnesium ions in a Penning trap to < 40 Kelvin. The Washington group cooled barium ions. The research from both groups served to illustrate the mechanical properties of light.^[11] Around this time, laser cooling techniques had allowed for temperatures lowered to around 40 kelvins.

Influenced by the Wineland's work on laser cooling ions, William Phillips applied the same principles to laser cool neutral atoms. In 1982, he published the first paper where neutral atoms were laser cooled.^[13] The process used is now known as the Zeeman slower and is a standard technique for slowing an atomic beam.

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 microkelvin when their Doppler cooling limit is 240 microkelvin,^[14] the new low was explained by the addition of more atomic states in combination to laser polarization. Previous conceptions of laser cooling were decided to have been too simplistic.^[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.^[16][15] The new state of matter, the Bose–Einstein condensate, was observed in 1995 by Eric Cornell, Carl Wieman, and Wolfgang Ketterle.^[17]

Laser cooling was primarily used to create ultracold atoms. For example, the experiments in quantum physics need to perform near absolute zero where unique quantum effects such as Bose–Einstein condensation can be observed. Laser cooling is also a primary tool in optical clock experiments.

Molecules

In 2010, a team at Yale successfully laser-cooled a diatomic molecule.^[18] In 2016, a group at MPQ successfully cooled formaldehyde to 420 μK via optoelectric Sisyphus cooling.^[19] In 2022, a group at Harvard successfully laser cooled and trapped CaOH to 720(40) μK in a magneto-optical trap.^[20]

Mechanical systems

In 2007, an MIT team successfully laser-cooled a macro-scale (1 gram) object to 0.8 K.^[21] In 2011, a team from the California Institute of Technology and the University of Vienna became the first to laser-cool a (10 μm x 1 μm) mechanical object to its quantum ground state.^[22]

Methods

The first example of laser cooling, and also still the most common method (so much so that it is still often referred to simply as 'laser cooling') is Doppler cooling.

Doppler cooling

Doppler cooling, which is usually accompanied by a magnetic trapping force to give a magneto-optical trap, is by far the most common method of laser cooling. It is used to cool low density gases down to the Doppler cooling limit, which for rubidium-85 is around 150 microkelvins.

In Doppler cooling, initially, the frequency of light is tuned slightly below an electronic transition in the atom. Because the light is detuned to the "red" (i.e., at lower frequency) of the transition, the atoms will absorb more photons if they move towards the light source, due to the Doppler effect. Thus if one applies light from two opposite directions, the atoms will always scatter more photons from the laser beam pointing opposite to their direction of motion. In each scattering event the atom loses a momentum equal to the momentum of the photon. 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. If the absorption and emission are repeated many times, the average speed, and therefore the kinetic energy of the atom, will be reduced. Since the temperature of a group of atoms is a measure of the average random internal kinetic energy, this is equivalent to cooling the atoms.

Anti-Stokes cooling

The idea for anti-Stokes cooling was first advanced by Pringsheim in 1929.^[23] While Doppler cooling lowers the translational temperature of a sample, anti-Stokes cooling decreases the vibrational or phonon excitation of a medium. This is accomplished by pumping a substance with a laser beam from a low-lying energy state to a higher one with subsequent emission to an even lower-lying energy state. The principal condition for efficient cooling is that the anti-Stokes emission rate to the final state be significantly larger than that to other states as well as the nonradiative relaxation rate. Because vibrational or phonon energy can be many orders of magnitude larger than the energy associated with Doppler broadening, the efficiency of heat removal per laser photon expended for anti-Stokes cooling can be correspondingly larger than that for Doppler cooling. The anti-Stokes cooling effect was first demonstrated by Djeu and Whitney in CO₂ gas.^[24] The first anti-Stokes cooling in a solid was demonstrated by Epstein et al. in 1980, in a ytterbium doped fluoride glass sample.^[25]

Potential practical applications for anti-Stokes cooling of solids include radiation balanced solid state lasers and vibration-free optical refrigeration.^[26][27]

Other methods

Other methods of laser cooling include:

• Sisyphus cooling^[28]
• Resolved sideband cooling
• Raman sideband cooling
• Velocity selective coherent population trapping (VSCPT)^[29]
• Gray molasses
• Optical molasses
• Cavity-mediated cooling^[30]
• Use of a Zeeman slower
• Electromagnetically induced transparency (EIT) cooling^[31]
• Anti-Stokes cooling in solids
• Polarization gradient cooling