Laser cooling includes several techniques where

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

s, and small mechanical systems are cooled with

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...

light. The directed energy of lasers is often associated with heating materials, e.g.

laser cutting Laser cutting is a technology that uses a laser to vaporize materials, resulting in a cut edge. While typically used for industrial manufacturing applications, it is now used by schools, small businesses, architecture, and hobbyists. Laser cutt ...

, so it can be counterintuitive that laser cooling often results in sample

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

s approaching

absolute zero Absolute zero is the lowest possible temperature, a state at which a system's internal energy, and in ideal cases entropy, reach their minimum values. The absolute zero is defined as 0 K on the Kelvin scale, equivalent to −273.15 ° ...

. It is a 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 clock Optical clocks are the most precise instruments ever developed. The precision of a clock is the smallest unit of time it can measure. Optical clocks reach record-breaking precision by counting oscillations of visible light, which oscillates up to ...

s. 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 A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...

(a particle of light). Atoms will be cooled in one dimension if they are illuminated by a pair of counter-propagating laser beams that are detuned below an atomic 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 Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described ...

. 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 Thermodynamic temperature, also known as absolute temperature, is a physical quantity which measures temperature starting from absolute zero, the point at which particles have minimal thermal motion. Thermodynamic temperature is typically expres ...

is proportional to the

variance In probability theory and statistics, variance is the expected value of the squared deviation from the mean of a random variable. The standard deviation (SD) is obtained as the square root of the variance. Variance is a measure of dispersion ...

in their velocity, therefore the lower the distribution of velocities, the lower the temperature of the particles.

History

Radiation pressure

Radiation pressure Radiation pressure (also known as light pressure) is mechanical pressure exerted upon a surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of ...

is the force that electromagnetic radiation exerts on matter. In 1873 Maxwell published his treatise on

electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

in which he predicted radiation pressure. The force was experimentally demonstrated for the first time by Lebedev and reported at a conference in Paris in 1900, and later published in more detail in 1901. Following Lebedev's measurements Nichols and Hull also demonstrated the force of radiation pressure in 1901, with a refined measurement reported in 1903. Atoms and molecules have bound states and transitions can occur between these states in the presence of light that is near the transition frequency. 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. This made it relatively easy to see the interaction of light with sodium atoms. In 1933,

Otto Frisch Otto Robert Frisch (1 October 1904 – 22 September 1979) was an Austrian-born British physicist who worked on nuclear physics. With Otto Stern and Immanuel Estermann, he first measured the magnetic moment of the proton. With his aunt, Lise M ...

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

s 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: Hänsch and Schawlow, and Wineland and 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 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.

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 below 40 K. The Washington group cooled barium ions. 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. The process used is now known as the

Zeeman slower In atomic physics, a Zeeman slower is a scientific instrument that is commonly used in atomic, molecular, and optical physics, atomic physics to slow and Cooling, cool a molecular beam, beam of hot atoms to speeds of several meters per second and ...

and is a standard technique for slowing an atomic beam. The 1997

Nobel Prize in Physics The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the ...

was awarded to

Claude Cohen-Tannoudji Claude Cohen-Tannoudji (; born 1 April 1933) is a French physicist. He shared the 1997 Nobel Prize in Physics with Steven Chu and William Daniel Phillips for research in methods of laser cooling and magnetic trap (atoms), trapping atoms. Currentl ...

Steven Chu Steven ChuWilliam 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

. The cooling processes were utilized to make

atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...

s more accurate and to improve spectroscopic measurements, and led to the observation of a new

state of matter In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and Plasma (physics), plasma. Different states are distinguished by the ways the ...

at ultracold temperatures. The new state of matter, the

Bose–Einstein condensate In condensed matter physics, a Bose–Einstein condensate (BEC) is a state of matter that is typically formed when a gas of bosons at very low Density, densities is cooled to temperatures very close to absolute zero#Relation with Bose–Einste ...

, was observed in 1995 by

Eric Cornell Eric Allin Cornell (born December 19, 1961) is an American physicist who, along with Carl E. Wieman, was able to synthesize the first Bose–Einstein condensate in 1995. For their efforts, Cornell, Wieman, and Wolfgang Ketterle shared the Nobe ...

, Carl Wieman, and

Wolfgang Ketterle Wolfgang Ketterle (; born 21 October 1957) is a German physicist and professor of physics at the Massachusetts Institute of Technology (MIT). His research has focused on experiments that trap and cool atoms to temperatures close to absolute zer ...

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 The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described t ...

. Laser cooling of antimatter has also been demonstrated, first in 2021 by the

ALPHA Alpha (uppercase , lowercase ) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter ''aleph'' , whose name comes from the West Semitic word for ' ...

collaboration on antihydrogen atoms. In 2024,

positronium Positronium (Ps) is a system consisting of an electron and its antimatter, anti-particle, a positron, bound together into an exotic atom, specifically an onium. Unlike hydrogen, the system has no protons. The system is unstable: the two part ...

, made up of an electron and a positron, was laser cooled to about 1K.

Molecules

Molecules are significantly more challenging to laser cool than atoms because molecules have vibrational and rotational degrees of freedom. These extra degrees of freedom result in more energy levels that can be populated from excited state decays, requiring more lasers compared to atoms to address the more complex level structure. Vibrational decays are particularly challenging because there are no symmetry rules that restrict the vibrational states that can be populated. In 2010, at team at Yale led by Dave DeMille successfully laser-cooled a

diatomic molecule Diatomic molecules () are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen () or oxygen (), then it is said to be homonuclear mol ...

. In 2016, a group at MPQ successfully cooled

formaldehyde Formaldehyde ( , ) (systematic name methanal) is an organic compound with the chemical formula and structure , more precisely . The compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde. It is stored as ...

to via optoelectric Sisyphus cooling. In 2022, a group at Harvard successfully laser cooled and trapped CaOH to in a

magneto-optical trap In atomic, molecular, and optical physics, a magneto-optical trap (MOT) is an apparatus which uses laser cooling and a spatially varying magnetic field to create a Magnetic trap (atoms), trap which can produce samples of Ultracold atom, cold neu ...

Mechanical systems

Starting in the 2000s, laser cooling was applied to small mechanical systems, ranging from small cantilevers to the mirrors used in the

LIGO The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Prior to LIG ...

observatory. These devices are connected to a larger substrate, such as a mechanical membrane attached to a frame, or they are held in optical traps, in both cases the mechanical system is a harmonic oscillator. Laser cooling reduces the random vibrations of the mechanical oscillator, removing thermal phonons from the system. In 2007, an MIT team successfully laser-cooled a macro-scale (1 gram) object to 0.8 K. In 2011, a team from the California Institute of Technology and the University of Vienna became the first to laser-cool a (10 μm × 1 μm) mechanical object to its quantum ground state.

Methods

The first realization of laser cooling and the most ubiquitous method for cooling atoms and molecules (so much so that it is often referred to simply as 'laser cooling'), is Doppler cooling.

Doppler cooling

Doppler cooling 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 Rubidium is a chemical element; it has Symbol (chemistry), symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have ...

(a popular choice in the field of atomic physics) is around 150

microkelvin List of orders of magnitude for temperature Detailed list for 100 K to 1000 K Most ordinary human activity takes place at temperatures of this order of magnitude. Circumstances where water naturally occurs in liquid form are shown in light g ...

. It is often often combined with a magnetic field gradient to realize a

. In Doppler cooling, initially, the frequency of light is tuned slightly below an

electronic transition In atomic physics and chemistry, an atomic electron transition (also called an atomic transition, quantum jump, or quantum leap) is an electron changing from one energy level to another within an atom or artificial atom. The time scale of a qua ...

in the

. Because the light is detuned to the "red" (i.e., at lower frequency) of the transition, the atoms will absorb more

s if they move towards the light source, due to the

. 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 In Newtonian mechanics, momentum (: momenta or momentums; more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. ...

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 In kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a non-negative scalar quantity. Intro ...

—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 In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...

of the atom, will be reduced. Since the

of a group of atoms is a measure of the average random internal kinetic energy, this is equivalent to cooling the atoms. When atoms are Doppler cooled in three dimensions, traditionally by 6 counter-propagating red-detuned laser beams, this is called optical molasses because the atoms move slowly, as if they are moving through molasses.

Sub-Doppler cooling

After Doppler cooling it is often helpful to cool atoms (or molecules) below their Doppler limit. This is accomplished with a variety sub-Doppler cooling techniques. Different atomic structures are amenable to different sub-Doppler cooling techniques. For example,

gray molasses Gray molasses is a method of Sub-Doppler cooling, sub-Doppler laser cooling of atoms. It employs principles from Sisyphus cooling in conjunction with a so-called "dark" state whose transition to the excited state is not addressed by the resonant la ...

is used with lithium and potassium because they have unresolved hyperfine structure in their excited states where polarization gradient cooling would not work. Sub-Doppler cooling methods include: * Sisyphus cooling * Polarization gradient cooling * Resolved sideband cooling * Raman sideband cooling * Velocity selective coherent population trapping (VSCPT) *

Gray molasses Gray molasses is a method of Sub-Doppler cooling, sub-Doppler laser cooling of atoms. It employs principles from Sisyphus cooling in conjunction with a so-called "dark" state whose transition to the excited state is not addressed by the resonant la ...

* Electromagnetically induced transparency (EIT) cooling

Other methods

Other laser cooling methods include: * Cavity-mediated cooling * Anti-Stokes cooling in solids * Photonic cooling – It is under development as a spot cooling system that can target areas of hundreds of microns in diameter. The laser(s) cool a plate less than one millimeter thick, made largely of

gallium arsenide Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a Zincblende (crystal structure), zinc blende crystal structure. Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...

Applications

Laser cooling is ubiquitous in the field of atomic physics. Reducing the random motion of atoms has several benefits, including the ability to trap atoms with optical or magnetic fields. 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. In a representative experiment a vapor of strontium atoms is generated in a hot oven that exit the oven as an atomic beam. After leaving the oven the atoms are Doppler cooled in two dimensions transverse to their motion to reduce loss of atoms due to divergence of the atomic beam. The atomic beam is then slowed and cooled with a Zeeman slower to optimize the atom loading efficiency into a magneto-optical trap (MOT), which Doppler cools the atoms, that operates on the with lasers at 461 nm. The MOT transitions from using light at 461 nm to using light at 689 nm to drive the , which is a narrow transition, to realize even colder atoms. The atoms are then transferred into an optical dipole trap where evaporative cooling gets them to temperatures where they can be effectively loaded into an optical lattice. Laser cooling is important for quantum computing efforts based on neutral atoms and trapped atomic ions. In an

ion trap An ion trap is a combination of electric field, electric and/or magnetic fields used to capture charged particles — known as ions — often in a system isolated from an external environment. Atomic and molecular ion traps have a number of a ...

Doppler cooling reduces the random motion of the ions so they form a well-ordered crystal structure in the trap. After Doppler cooling the ions are often cooled to their motional ground state to reduce decoherence during quantum gates between ions. Photonic cooling is under development for use to cool chip hotspots in data centers.

Equipment

Laser cooling atoms requires scientific equipment that when assembled forms a cold atom machine. Such machines consist of two parts: a vacuum chamber which houses the laser cooled atoms and the laser systems used for cooling, as well as for preparing and manipulating atomic states and detecting the atoms. Laser cooling molecules generally requires more lasers and optical modulators (such as electro-optic and acousto-optic) to address the more complex molecular structure. Mechanical systems also need a vacuum system as the damping from background gases quickly equilibrates them with the gas's temperature. Mechanical systems usually need only one laser which is chosen for its reliability and coherence time, such as Nd:YAG lasers or Fiber lasers, as the mechanical devices are reflective over a very wide range of wavelengths.

Vacuum system

In order for atoms to be laser cooled, the atoms cannot collide with room temperature background gas particles. Such collisions will drastically heat the atoms, and knock them out of weak traps. Acceptable collision rates for cold atom machines typically require vacuum pressures at 10⁻⁹ Torr, and very often hundreds or even thousands of times lower pressures are necessary. To achieve these low pressures, a vacuum chamber is needed. The vacuum chamber typically includes windows so that the atoms can be addressed with lasers (e.g. for laser cooling) and light emitted by the atoms or absorption of light be the atoms can be detected. The vacuum chamber also requires an atomic source for the atom(s) to be laser cooled. The atomic source is generally heated to produce thermal atoms that can be laser cooled. For ion trapping experiments the vacuum system must also hold the ion trap, with the appropriate electric feedthroughs for the trap. Neutral atom systems very often employ a

Magneto-optical trap In atomic, molecular, and optical physics, a magneto-optical trap (MOT) is an apparatus which uses laser cooling and a spatially varying magnetic field to create a Magnetic trap (atoms), trap which can produce samples of Ultracold atom, cold neu ...

(MOT) as one of the early stages in collecting and cooling atoms. For a MOT typically magnetic field coils are placed outside of the vacuum chamber to generate magnetic field gradients for the MOT.

Lasers

The

required for cold atom machines are entirely dependent on the choice of atom. Each atom has unique electronic transitions at very distinct wavelengths that must be driven for the atom to be laser cooled. 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. The light for rubidium can be generated from a signal laser at 780 nm and an Electro-optic modulator. Generally tens of mW (and often hundreds of mW to cool significantly more atoms) is used to cool neutral atoms. 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. The strontium ion, for example requires light at both 422 nm and 1092 nm in order to be Doppler cooled. Because of the small Doppler shifts involved with laser cooling, very narrow lasers, order of a few MHz, are required for laser cooling. Such lasers are generally stabilized to spectroscopy reference cells, optical cavities, or sometimes wavemeters so the laser light can be precisely tuned relative to the atomic transitions. Ytterbium_Lattice_Double_Clock_with_Photons_at_NIST

Ytterbium_Lattice_Double_Clock_with_Photons_at_NIST

References

Additional sources

* * * * * PhysicsWorld series of articles by

Chad Orzel Chad Orzel is a professor of physics and science author, noted for his books ''How to Teach Quantum Physics to Your Dog'', which has been translated into 9 languages, and ''How to Teach Relativity to Your Dog''. Chad as a science communicator is a ...

: *
Cold: how physicists learned to manipulate and move particles with laser cooling
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Colder: how physicists beat the theoretical limit for laser cooling and laid the foundations for a quantum revolution
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Coldest: how a letter to Einstein and advances in laser-cooling technology led physicists to new quantum states of matter
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