Laser cooling

With laser cooling methods are called, with which gas or atomic beams are cooled by irradiation with laser light. This exploits that a pulse can be transmitted through light.

The basic idea

The temperature of a gas is expressed in the ( disordered ) motion of the atoms, see kinetic theory of gases. The higher the mean velocity of the atoms in a gas, the hotter it. The speed of the atoms can be reduced by skillful bombardment with light quanta ( photons). Does a photon interacts with an atom, it can be absorbed, it is a shell electron in an excited state. This may after a time " disintegrate " (spontaneous emission), while a photon is emitted in a random direction (see resonance fluorescence). The atom is replaced, due to conservation of momentum, at every absorption and emission of a small recoil. Irradiating now atoms with a laser, then each individual atom in succession sprinkle a very large number of photons. Here, the recoil always goes in the absorption in the same direction and therefore has on average over many photons scatter a large effect, while the emission occurring during recoil always goes in a different direction and cancels out over time. By exploiting the Doppler effect can be achieved, that atoms are irradiated with laser light from all directions, mainly absorb photons from the beam to which they get around. The resultant force is the direction of movement of the atoms in opposite directions and thereby brakes off. The average velocity decreases over time, the gas is colder.

Doppler cooling

Cooling in the two-state system

We used here a specific transition in the atomic spectrum (often a hyperfine transition), which can be regarded as a simple two-state system. Irradiation of resonant photons ( eg laser light ), the initial or ground state ( see Bra- Ket notation ) are brought into the excited state. The excited state will fall back after a short time in two possible ways to the ground state: on the one stimulated emission is possible, on the other hand spontaneous emission. For both processes from the excited atom a photon is re-emitted.

In the event of the stimulated emission is no net momentum transfer is made to the atom, as a photon of the same energy is radiated in the direction of the incident photon. If, however, the atom spontaneously from the excited state to the ground state, so the direction of re-emission is random. Since the photon had a preferred direction in the radiation, the atoms are slowly accelerated / decelerated in this direction. Beam is controlled laser light from all directions in space one, so you can slow down all the atoms - no matter which direction they are moving. This is done using three pairs of two counter-rotating laser beams that are perpendicular to each other.

As "hot" moving atoms, acting on it is not the frequency of the light in the laboratory system, but a shifted due to the Doppler effect frequency. So one has from the resonance frequency of the transition radiate detuned laser light to excite these absorption or Reemissionszyklus. With a specific mood of the laser light are selected so atoms of a certain speed. This selection is not " sharp " as the atom thus has an absorption profile in the form of a Lorentzian curve and a certain line width. At a redshift of the incident light, the process acts only on atoms to move on to the laser beam.

However, using in one space dimension two opposing, rotverstimmte laser beams, this results in a velocity-dependent force acting on all atoms with v ≠ 0, and this slows down (see chart at right). To concerned atoms in all three spatial dimensions (x, y, z) to cool, you is six - in pairs opposite - laser beams. The pairs are thereby mutually perpendicular. This configuration of the laser as optical molasses ( engl. optical molasses ) because it acts like a viscous liquid, and so slows down atoms.

The limit of this cooling process is given by the so-called Doppler temperature and is generally in the range of a few 100 μK. The reason for this lower limit are two heating processes that occur at Doppler cooling always. The first is caused by the oscillatory motion of the atoms due to the spontaneous emission. The second is caused by the shot noise of the incident laser light. For rubidium atoms, the Doppler temperature is about 140 μK.

This method makes it possible indeed to cool the atoms concerned, it can not hold in one place though. Due to the residual velocity ( Doppler limit ) they can diffuse out of the cooling region. That's why we extended the method for magneto- optical trap (MOT ), in which the atoms are still caught in a magnetic field and remain localized so locally. With it, you can also reach temperatures that are lower than the Doppler limit.

With the help of the Zeeman effect can be the atomic transition in a magnetic field detuning so that the Doppler shift is compensated. The so-called Zeeman Slower uses this mechanism, to brake a beam of fast atoms.

Real atoms and return pump

For cooling, it requires a system of two states, so that a transition is excited by excitation with photons, which can decay only to the initial state. In the two-state system, it is always given. However, one used in laboratory systems that are just no perfect two-state systems, and so there is always the chance that atoms decay into a state where they no longer resonate are to the incident light ( dark state, Eng. Dark state) and so from the lost cycle. In the picture you can see the rubidium transition, which is used for cooling, as it is in the near infrared ( 780 nm) and laser of this wavelength are readily available. The excited state of the cooling transition ( shown in blue ) breaks down once in 1,000 cycles in the lower 2S1/2-Zustand F = 2 instead of F = 3, from which he ( often referred to loosely as " repumper " ) with a return pump laser must be retrieved. The decay happens not made directly, because the transition F = 4 is banned by F = 2 (F = 0, ± 1, cf selection rule ), but the laser excited due to the rather large line width of the atomic transition (about 10 MHz) and the 2P3 / 2, F = 3 to state which can then decay into the lower ground state. The return pump laser lifts atoms from the lower ground state to 2P3 / 2, F = 3 (see the drawing), from which they can then return to scheduled cycle again (thin red drawn- decay).

One uses this fact to the atoms condense in a so-called strongly dark spot -MOT, than would otherwise be possible.

Subdopplerkühlen

There are several ways in which one atom or to cool under the Doppler temperature. In a magneto-optical trap a moving atom in an optical standing wave is periodically between different magnetic sublevels back and hergepumpt, with kinetic is converted into potential energy. This so-called Sisyphuskühlung is more effective than the Doppler cooling at low speeds, and can cool the atoms to recoil limit. This limit corresponds to the energy absorbed by a stationary atom when it absorbs a photon.

Under the recoil limit atoms can be cooled by ensuring that fails to interact dormant atoms with laser light. This is for example achieved in that the atoms are cooled in a speed-dependent dark state. This method is called by the English VSCPT velocity selective coherent population trapping.

Since the Doppler temperature depends only on spontaneous scattering rate, so the lifetime of the excited state, it is possible to achieve extremely low temperatures in some atoms, such as alkaline earth metals, and by a second Doppler cooling step at a very durable intersystem crossing.

To achieve even lower temperatures, where one can observe the quantum mechanical degeneration of the atomic gas, the method of the evaporative cooling is employed.

Sideband cooling

Sideband cooling is a cooling of the Doppler related method of cooling the laser, but which is not applied to most atomic gases, but on individual ions in an ion trap. Here use is made that the particles can not move freely in such a case, but can only occupy discrete vibrational states. In this case, the ions can not only absorb light at its resonant frequency, but also at frequencies whose distance from the resonance frequency just corresponds to the energy distance between adjacent vibration levels. This is referred to as blue and red sidebands. These sidebands are similar to the Doppler shift in the case of laser cooling of free atoms. In the simplest case, an ion on the red sideband is excited, while his vibrational excitation is reduced by one quantum. The excited state of the ion decomposes spontaneously, wherein transitions of the resonant frequency or at one of the sidebands are also possible. Then, the ion is again in the ground state, but ( most likely ) Vibration energy is lost. This cycle is repeated in the ideal case, as long as the ion reaches the lowest vibrational state. This can be experimentally seen from the fact that no fluorescence occurs more on the red sideband. In practice transitions are generally used optical, in which the excited state has a very long service life. Therefore, it is not possible to wait for the spontaneous decay of the excited state. It then uses one or more return pump lasers, which can return more quickly to the ground state of the ion through another intermediate state.

History

In 1997, the Nobel Prize in Physics for the development of laser cooling to Steven Chu, Claude Cohen- Tannoudji and William D. Phillips was awarded.