Laser cooling
Encyclopedia
Laser cooling refers to the number of techniques in which atomic and molecular samples are cooled through the interaction with one or more laser
light fields. The first example of laser cooling, and also still the most common method of laser cooling (so much so that it is still often referred to as 'laser cooling') is Doppler cooling
.
Other methods of laser cooling include:
, is by far the most common method of laser cooling. It is used to cool low density gasses down to the Doppler cooling limit
, which for Rubidium 85 is around 150 microkelvin. As Doppler cooling requires a very particular energy level structure, known as a closed optical loop, the method is limited to a small handful of elements.
In Doppler cooling, the frequency of light is tuned slightly below an electronic transition in the atom
. Because the light is detuned
to the "blue" (i.e. at higher frequency) of the transition, the atoms will absorb more photon
s 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, emits a photon spontaneously, it will be kicked by the same amount of momentum but in a random direction. The result of the absorption and emission process is to reduce the speed of the atom, provided its initial speed is larger than the recoil velocity
from scattering a single photon. If the absorption and emission are repeated many times, the mean velocity, and therefore the kinetic energy
of the atom will be reduced. Since the temperature
of an ensemble of atoms is a measure of the random internal kinetic energy, this is equivalent to cooling the atoms.
and photoluminescent
upconversion
have been studied as means to achieve the same effects. In many of these, the coherence of the laser light is not essential to the process, but lasers are typically used to achieve a high irradiance
.
(−273.15°C, −459.67°F). This is done to observe the unique quantum effects that can only occur at this heat level. Generally, laser cooling has been only used on the atomic level to cool down elements. This may soon change, as a new breakthrough in the technology has successfully cooled a macro-scale object to near absolute zero.
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
light fields. The first example of laser cooling, and also still the most common method of laser cooling (so much so that it is still often referred to as 'laser cooling') is Doppler cooling
Doppler cooling
Doppler cooling is a mechanism that can be used to trap and cool atoms. The term is sometimes used synonymously with laser cooling, though laser cooling includes other techniques.-History:...
.
Other methods of laser cooling include:
- Sisyphus coolingSisyphus coolingSisyphus cooling is a mechanism through which atoms can be cooled using laser beams below the temperatures expected to be achieved by Doppler cooling. It comes about as a result of a polarization gradient created by two counter-propagating laser beams with orthogonal polarization...
- Resolved sideband coolingResolved sideband coolingResolved sideband cooling is a laser cooling technique that can be used to cool strongly trapped atoms to the quantum ground state of their motion. The atoms are usually precooled using the Doppler laser cooling...
- Velocity selective coherent population trapping (VSCPT)
- Anti-Stokes inelastic light scattering (typically in the form of fluorescenceFluorescenceFluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
or Raman scatteringRaman scatteringRaman scattering or the Raman effect is the inelastic scattering of a photon. It was discovered by Sir Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids, and by Grigory Landsberg and Leonid Mandelstam in crystals....
) - Cavity mediated cooling
- Sympathetic coolingSympathetic coolingSympathetic cooling is a process in which particles of one type cool particles of another type.Typically, atomic ions that can be directly laser cooled are used to cool nearby ions or atoms, by way of their mutual Coulomb interaction. This technique allows cooling of ions and atoms that can't be...
- Use of a Zeeman slowerZeeman slowerA Zeeman slower is a scientific apparatus that is commonly used in experimental atomic, molecular, and optical physics to slow a beam of atoms or molecules from initial speeds on the order of 500 m/s-1000 m/s to final speeds on the order of 10 m/s...
Doppler cooling
Doppler cooling, which is usually accompanied by a magnetic trapping force to give a magneto-optical trapMagneto-optical trap
A magneto-optical trap is a device that uses both laser cooling with magneto-optical trapping in order to produce samples of cold, trapped, neutral atoms at temperatures as low as several microkelvins, two or three times the recoil limit.By combining the small momentum of a single photon with a...
, is by far the most common method of laser cooling. It is used to cool low density gasses down to the Doppler cooling limit
Doppler cooling limit
Doppler temperature is the minimum temperature achievable with Doppler cooling, one of the methods of laser cooling.When a photon is absorbed by an atom moving in the opposite direction, its velocity is decreased according to the laws of momentum conservation. Accordingly, when a photon is emitted...
, which for Rubidium 85 is around 150 microkelvin. As Doppler cooling requires a very particular energy level structure, known as a closed optical loop, the method is limited to a small handful of elements.
In Doppler cooling, the frequency of light is tuned slightly below an electronic transition in the atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
. Because the light is detuned
Laser detuning
In optical physics, laser detuning is the tuning of a laser to a frequency that is slightly off from a quantum system's resonant frequency. Lasers can be detuned in the lab frame so that they are Doppler shifted to the resonant frequency in a moving system, which allows lasers to affect only...
to the "blue" (i.e. at higher frequency) of the transition, the atoms will absorb more photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s if they move towards the light source, due to the Doppler effect
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
. Thus if one applies light from two opposite directions, the atoms will always scatter more photons from the laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
beam pointing opposite to their direction of motion. In each scattering event the atom loses a momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
equal to the momentum of the photon. If the atom, which is now in the excited state, emits a photon spontaneously, it will be kicked by the same amount of momentum but in a random direction. The result of the absorption and emission process is to reduce the speed of the atom, provided its initial speed is larger than the recoil velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
from scattering a single photon. If the absorption and emission are repeated many times, the mean velocity, and therefore the kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of the atom will be reduced. Since the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
of an ensemble of atoms is a measure of the random internal kinetic energy, this is equivalent to cooling the atoms.
Other methods of laser cooling
Several somewhat similar processes are also referred to as laser cooling, in which photons are used to pump heat away from a material and thus cool it. The phenomenon has been demonstrated via anti-Stokes fluorescence, and both electroluminescent upconversionPhoton upconversion
Photon upconversion is a process in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength. It is a anti-Stokes type emission. An example is the conversion of infrared light to visible light. Materials by which...
and photoluminescent
Photoluminescence
Photoluminescence is a process in which a substance absorbs photons and then re-radiates photons. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon...
upconversion
Photon upconversion
Photon upconversion is a process in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength. It is a anti-Stokes type emission. An example is the conversion of infrared light to visible light. Materials by which...
have been studied as means to achieve the same effects. In many of these, the coherence of the laser light is not essential to the process, but lasers are typically used to achieve a high irradiance
Irradiance
Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...
.
Uses
Laser cooling is primarily used for experiments in Quantum Physics to achieve temperatures of near absolute zeroAbsolute zero
Absolute zero is the theoretical temperature at which entropy reaches its minimum value. The laws of thermodynamics state that absolute zero cannot be reached using only thermodynamic means....
(−273.15°C, −459.67°F). This is done to observe the unique quantum effects that can only occur at this heat level. Generally, laser cooling has been only used on the atomic level to cool down elements. This may soon change, as a new breakthrough in the technology has successfully cooled a macro-scale object to near absolute zero.
See also
- Resolved sideband coolingResolved sideband coolingResolved sideband cooling is a laser cooling technique that can be used to cool strongly trapped atoms to the quantum ground state of their motion. The atoms are usually precooled using the Doppler laser cooling...
- optical tweezersOptical tweezersOptical tweezers are scientific instruments that use a highly focused laser beam to provide an attractive or repulsive force , depending on the refractive index mismatch to physically hold and move microscopic dielectric objects...
- Sympathetic coolingSympathetic coolingSympathetic cooling is a process in which particles of one type cool particles of another type.Typically, atomic ions that can be directly laser cooled are used to cool nearby ions or atoms, by way of their mutual Coulomb interaction. This technique allows cooling of ions and atoms that can't be...
- Mössbauer effectMössbauer effectThe Mössbauer effect, or recoilless nuclear resonance fluorescence, is a physical phenomenon discovered by Rudolf Mössbauer in 1958. It involves the resonant and recoil-free emission and absorption of γ radiation by atomic nuclei bound in a solid...
- Mössbauer spectroscopy
- Magneto-optical trapMagneto-optical trapA magneto-optical trap is a device that uses both laser cooling with magneto-optical trapping in order to produce samples of cold, trapped, neutral atoms at temperatures as low as several microkelvins, two or three times the recoil limit.By combining the small momentum of a single photon with a...
- Steven ChuSteven ChuSteven Chu is an American physicist and the 12th United States Secretary of Energy. Chu is known for his research at Bell Labs in cooling and trapping of atoms with laser light, which won him the Nobel Prize in Physics in 1997, along with his scientific colleagues Claude Cohen-Tannoudji and...
- Timeline of low-temperature technologyTimeline of low-temperature technologyThe following is a timeline of low-temperature technology and cryogenic technology .-16th century BCE – 17th century CE :...