Optical molasses is a laser cooling technique that can cool neutral atoms to as low as a few microkelvins, depending on the atomic species. An optical molasses consists of 3 pairs of counter-propagating orthogonally polarized laser beams intersecting in the region where the atoms are present. The main difference between an optical molasses (OM) and a magneto-optical trap (MOT) is the absence of magnetic field in the former. Unlike a MOT, an OM provides only cooling and no trapping.
When laser cooling was proposed in 1975, a theoretical limit on the lowest possible temperature was predicted. [1] Known as the Doppler limit, , this was given by the lowest possible temperature attainable considering the cooling of two-level atoms by Doppler cooling and the heating of atoms due to momentum diffusion from the scattering of laser photons. Here is the natural line-width of the atomic transition, is the reduced Planck constant, and is the Boltzmann constant.
The first experimental realization of optical molasses was achieved in 1985 by Chu et al. at AT&T Bell Laboratories. [2] The authors measured laser cooling of neutral sodium atoms down to the theoretical Doppler cooling limit by observing the fluorescence of a hot atomic beam. By temporarily switching off the laser beams for a fixed time interval, the authors firstly measured the average kinetic energy of the atoms by a time-of-flight technique. The fraction of atoms that left the region while it was in the dark was measured by comparing the brightness of the fluorescence before and after the turnoff. Then velocity distribution and temperature were measured by estimating the dependence of this fraction on the light-off time. The kinetic temperature they obtained was T ≈ 240 μK, not very different from the Doppler cooling limit in the two-level approximation. The size of the optical molasses region was a limiting factor.
Experiments at the National Institute of Standards and Technology in Gaithersburg found the temperature of cooled atoms to be well below the theoretical limit. [3] In 1988, Lett et al. [3] directed sodium atoms through an optical molasses and found the temperatures to be as low as ~40 μk, 6 times lower than the expected 240 μk Doppler cooling limit. Other unexpected properties found in other experiments [4] included significant unexpected insensitivity to laser alignment of the counter-propagating beams.
These unexpected observations led to the development of more sophisticated models [5] of laser cooling that took into account the Zeeman and hyperfine sublevels of the atomic structure. The dynamics of optical pumping between these sublevels allow the cooling of atoms below the Doppler limit.
The best explanation of the phenomenon of optical molasses is based on the principle of polarization gradient cooling. [6] For one-dimensional optical molasses: Suppose two laser beams approach an atom from opposite directions. Counterpropagating beams of circularly polarized light cause a standing wave, where the light polarization is linear but the direction rotates along the direction of the beams at a very fast rate. Atoms moving in the spatially varying linear polarization have a higher probability density of being in a state that is more susceptible to absorption of light from the beam coming head-on, rather than the beam from behind. This results in a velocity-dependent damping force [7] where The variable is the reduced Planck constant, is the saturation intensity, is the laser detuning, and is the linewidth of the atom-cooling transition. For sodium, the cooling (cycling) transition is the transition, driven by laser light at 589 nm.
The optical molasses can reduce the atom temperature to the recoil limit is set by the energy of the photon emitted in the decay from the J′ to J state, where the J state is the ground-state angular momentum, and the J′ state is the excited-state angular momentum. This temperature is given by though practically the limit is a few times this value because of the extreme sensitivity to external magnetic fields in this cooling scheme. Atoms typically reach temperatures on the order of microkelvins, as compared to the doppler limit μK.
The one-dimensional optical molasses can be extended to three dimensions with six counter-propagating laser beams. The total force is the sum from each beam. For example, a study [8] using cesium atoms achieved temperatures as low as ~3 μK, approximately 40 times below the Doppler limit and only slightly above the recoil temperature limit of Cs. The temperature obtained varies with the configuration of the laser polarization and are all higher than the theoretical estimate. Thus the extension has been proven to be effective, despite a few caveats. In 3D experiments, the transverse nature of light leads to the limitation that there will always be polarization gradients. The atoms also see different gradients along different directions, and they may change dramatically during the atom's diffusive movement in the molasses. [9] The trajectories are not straight either, but severely affected by the cooling process. [10] Quantum treatments are needed due to these limitations.
An optical molasses slows down the atoms but does not provide any trapping force to confine them spatially. A magneto-optical trap employs a 3-dimensional optical molasses along with a spatially varying magnetic field to slow down and confine the atoms.
Spontaneous emission is the process in which a quantum mechanical system transits from an excited energy state to a lower energy state and emits a quantized amount of energy in the form of a photon. Spontaneous emission is ultimately responsible for most of the light we see all around us; it is so ubiquitous that there are many names given to what is essentially the same process. If atoms are excited by some means other than heating, the spontaneous emission is called luminescence. For example, fireflies are luminescent. And there are different forms of luminescence depending on how excited atoms are produced. If the excitation is effected by the absorption of radiation the spontaneous emission is called fluorescence. Sometimes molecules have a metastable level and continue to fluoresce long after the exciting radiation is turned off; this is called phosphorescence. Figurines that glow in the dark are phosphorescent. Lasers start via spontaneous emission, then during continuous operation work by stimulated emission.
Ionization is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule is called an ion. Ionization can result from the loss of an electron after collisions with subatomic particles, collisions with other atoms, molecules, electrons, positrons, protons, antiprotons and ions, or through the interaction with electromagnetic radiation. Heterolytic bond cleavage and heterolytic substitution reactions can result in the formation of ion pairs. Ionization can occur through radioactive decay by the internal conversion process, in which an excited nucleus transfers its energy to one of the inner-shell electrons causing it to be ejected.
Laser cooling, sometimes also referred to as Doppler 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 a routine step in many atomic physics experiments where the laser-cooled atoms are then subsequently manipulated and measured, or in technologies, such as atom-based quantum computing architectures. Laser cooling relies on the change in momentum when an object, such as an atom, absorbs and re-emits a photon. 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, therefore the lower the distribution of velocities, the lower temperature of the particles.
Optical tweezers are scientific instruments that use a highly focused laser beam to hold and move microscopic and sub-microscopic objects like atoms, nanoparticles and droplets, in a manner similar to tweezers. If the object is held in air or vacuum without additional support, it can be called optical levitation.
Evaporative cooling is an atomic physics technique to achieve high phase space densities which optical cooling techniques alone typically can not reach.
Resolved sideband cooling is a laser cooling technique allowing cooling of tightly bound atoms and ions beyond the Doppler cooling limit, potentially to their motional ground state. Aside from the curiosity of having a particle at zero point energy, such preparation of a particle in a definite state with high probability (initialization) is an essential part of state manipulation experiments in quantum optics and quantum computing.
In condensed matter physics and atomic physics, the recoil temperature is a fundamental lower limit of temperature attainable by some laser cooling schemes. When an atom decays from an excited electronic state at rest to a lower energy electronic state by the spontaneous emission of a photon, due to conservation of momentum, the atom gains momentum equivalent to the momentum of the photon. This kinetic energy gain corresponds to the recoil temperature of the atom. The recoil temperature is
An optical lattice is formed by the interference of counter-propagating laser beams, creating a spatially periodic polarization pattern. The resulting periodic potential may trap neutral atoms via the Stark shift. Atoms are cooled and congregate at the potential extrema. The resulting arrangement of trapped atoms resembles a crystal lattice and can be used for quantum simulation.
Doppler cooling is a mechanism that can be used to trap and slow the motion of atoms to cool a substance. The term is sometimes used synonymously with laser cooling, though laser cooling includes other techniques.
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 trap which can produce samples of cold neutral atoms. Temperatures achieved in a MOT can be as low as several microkelvins, depending on the atomic species, which is two or three times below the photon-recoil limit. However, for atoms with an unresolved hyperfine structure, such as 7Li, the temperature achieved in a MOT will be higher than the Doppler cooling limit.
In condensed matter physics, an ultracold atom is an atom with a temperature near absolute zero. At such temperatures, an atom's quantum-mechanical properties become important.
In spectroscopy, the Autler–Townes effect, is a dynamical Stark effect corresponding to the case when an oscillating electric field is tuned in resonance to the transition frequency of a given spectral line, and resulting in a change of the shape of the absorption/emission spectra of that spectral line. The AC Stark effect was discovered in 1955 by American physicists Stanley Autler and Charles Townes.
The manipulation of atoms using optical fields is a vital and fundamental area of research within the field of atomic physics. This research revolves around leveraging the distinct characteristics of laser light and coherent optical fields to achieve precise control over various aspects of atomic systems. These aspects encompass regulating atomic motion, positioning atoms, manipulating internal states, and facilitating intricate interactions with neighboring atoms and photons. The utilization of optical fields provides a powerful toolset for exploring and understanding the quantum behavior of atoms and opens up promising avenues for applications in atomic, molecular, and optical physics.
In ultra-low-temperature physics, Sisyphus cooling, the Sisyphus effect, or polarization gradient cooling involves the use of specially selected laser light, hitting atoms from various angles to both cool and trap them in a potential well, effectively rolling the atom down a hill of potential energy until it has lost its kinetic energy. It is a type of laser cooling of atoms used to reach temperatures below the Doppler cooling limit. This cooling method was first proposed by Claude Cohen-Tannoudji in 1989, motivated by earlier experiments which observed sodium atoms cooled below the Doppler limit in an optical molasses. Cohen-Tannoudji received part of the Nobel Prize in Physics in 1997 for his work. The technique is named after Sisyphus, a figure in the Greek mythology who was doomed, for all eternity, to roll a stone up a mountain only to have it roll down again whenever he got it near the summit.
In atomic physics, a Zeeman slower is a scientific instrument that is commonly used in atomic physics to slow and cool a beam of hot atoms to speeds of several meters per second and temperatures below a kelvin. The gas-phase atoms used in atomic physics are often generated in an oven by heating a solid or liquid atomic sample to temperatures where the vapor pressure is high enough that a substantial number of atoms are in the gas phase. These atoms effuse out of a hole in the oven with average speeds on the order of hundreds of m/s and large velocity distributions. The Zeeman slower is attached close to where the hot atoms exit the oven and are used to slow them to less than 10 m/s (slowing) with a very small velocity spread (cooling).
In atomic physics, Raman cooling is a sub-recoil cooling technique that allows the cooling of atoms using optical methods below the limitations of Doppler cooling, Doppler cooling being limited by the recoil energy of a photon given to an atom. This scheme can be performed in simple optical molasses or in molasses where an optical lattice has been superimposed, which are called respectively free space Raman cooling and Raman sideband cooling. Both techniques make use of Raman scattering of laser light by the atoms.
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. When used as a noun, the laser detuning is the difference between the resonance frequency of the system and the laser's optical frequency. Lasers tuned to a frequency below the resonant frequency are called red-detuned, and lasers tuned above resonance are called blue-detuned. This technique is essential in many AMO physics experiments and associated technologies, as it allows the manipulation of light-matter interactions with high precision. Detuning has use cases in research fields including quantum optics, laser cooling, and spectroscopy. It is also fundamental to many modern and emerging atomic and quantum technologies, such as atomic clocks, quantum computers, and quantum sensors. By adjusting the detuning, researchers and engineers can control absorption, emission, and scattering processes, making it a versatile tool in both fundamental and applied physics.
Sub-Doppler cooling is a class of laser cooling techniques that reduce the temperature of atoms and molecules below the Doppler cooling limit. In experiment implementation, Doppler cooling is limited by the broad natural linewidth of the lasers used in cooling. Regardless of the transition used, however, Doppler cooling processes have an intrinsic cooling limit that is characterized by the momentum recoil from the emission of a photon from the particle. This is called the recoil temperature and is usually far below the linewidth-based limit mentioned above. By laser cooling methods beyond the two-level approximations of atoms, temperature below this limit can be achieved.
Gray molasses is a method of 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 lasers. Ultracold atomic physics experiments on atomic species with poorly-resolved hyperfine structure, like isotopes of lithium and potassium, often utilize gray molasses instead of Sisyphus cooling as a secondary cooling stage after the ubiquitous magneto-optical trap (MOT) to achieve temperatures below the Doppler limit. Unlike a MOT, which combines a molasses force with a confining force, a gray molasses can only slow but not trap atoms; hence, its efficacy as a cooling mechanism lasts only milliseconds before further cooling and trapping stages must be employed.
Polarization gradient cooling, or Sisyphus cooling, is a technique in laser cooling of atoms by dampening the motion of the trapped particles via photon momentum. It was proposed to explain the experimental observation of cooling below the Doppler limit observed in cesium atom-related laser cooling experiments in 1985. Shortly after the theory was introduced, experiments were performed that verified the theoretical predictions. While Doppler cooling allows atoms to be cooled to hundreds of microkelvin, PG cooling allows atoms to be cooled to a few microkelvin or less.
We present two cooling mechanisms that lead to temperatures well below the Doppler limit. These mechanisms are based on laser polarization gradients and work at low laser power when the optical-pumping time between different ground-state sublevels becomes long.
