Sisyphus cooling

Sisyphuskühlung ( rarely Sisyphus cooling) or Polarisationsgradientenkühlung is in laser spectroscopy, a special form of laser cooling can be achieved with lower temperatures than the Doppler temperature.

History

This cooling method was first proposed in 1989 by Claude Cohen- Tannoudji, for which he received the Nobel Prize for Physics in 1997. The name is derived from the Greek hero Sisyphus, who had to be carried up the same mountain a stone again and again.

Construction

In order to implement a linearly polarized laser Sisyphuskühlung in the opposite direction is overlaid with rotated by 90 ° polarization with itself. Thus a standing wave whose polarization is spatially periodic changes and alternate between forms -, - and - polarization changes. This condition is also referred to as linlin arrangement ( linear vertical - linear). To be cooled atoms have to move in and along the laser beam.

Physical Basics

The areas of the laser light induce with a transition, whereas the light causes a transition with. Since the spontaneous emission after excitation in the middle does not change (), the atoms are thus sent from one state degeneracy of the ground state to the other. As the atom moves along the ray, then it reaches the other polarization and is converted back to its original state. The state of the atom always oscillate now between the two degeneracies of the ground state.

Furthermore, the Stark effect, which is caused by the electric field of the laser on the atom is a function of the transition probability between the energy levels and the upset. This means that the energy levels of the atom with large transition probability energy levels farther away from each other than the small transition probability. In Rotverstimmung the energy of the ground state decreases. Depending on which of the two ground states is the atom - Since also changes the polarization, also the transition probability varies.

An atom in the ground state, which moves along the laser must start against the potential of standing wave. It is gaining potential energy, but loses kinetic energy, corresponding to a cooling. The maximum of the wave crest, the probability of absorption of light is at a maximum, the atom is therefore highly likely set in the excited state, by spontaneous emission, it then proceeds to the initial state over and the process begins, this time with a light from the front.

The theoretical limit of the temperature at Sisyphuskühlung is located in the one-dimensional case with the recoil temperature

With the momentum of the emitted photons, the Boltzmann constant and the atomic mass. These temperatures are in the range of micro- Kelvin. Since this temperature depends on the photon momentum, can be reached by larger Rotverstimmung of the laser lower temperatures. The detuning may, however, not be too large, otherwise the total cooling capacity decreases, as the Anregungswahrscheinschlichkeit increases with the detuning from.

Postgraduate

• Wolfgang Demtröder: laser spectroscopy. 5th edition. Springer, Kaiserslautern 2007, ISBN 978-3-540-33792-8.

• Tobias Müther: Evaporative cooling in optical Dipolpotentialen