Magneto-optical trap

A magneto- optical trap (English magneto- optical trap MOT ) is a tool of atomic physics, which is used for cooling and trapping of neutral atoms. Cooling means here deceleration of the atoms, as the temperature of a single atom, that is expressed by its kinetic energy its speed. This means: If there is a " large number " of particles (atoms, molecules, etc.) in thermal equilibrium, the average kinetic energy of a particle can be expressed by a temperature.

In contrast to the storage of the charged particles (ions), which can be captured by the electric and magnetic fields in a Paul or Penning trap is required for neutral atoms, the optical power, as it is, regardless of load. An MOT consists of six pairs of counter-propagating laser, make the laser cooling. In addition, you need a magnetic field to keep the cooled atoms in place. Here, the magnetic field causes a Zeeman splitting of the nuclear levels with the laser beams, in conjunction to a position-dependent force.

The magneto - optical trap was the beginning of the experimental research on Bose -Einstein condensation and is used in many experiments for the study of cold atoms.

Principle of operation

Cool

Atoms are cooled according to the principle of the laser cooling system. That is, the transmitted photons from the laser its pulse to the atom which is in this case stimulated for energy. This excited state decays after spontaneous or induced. However, while the direction of the absorbed and emitted photons are identical for the stimulated emission, the direction of the emitted photon is independent from that of the absorbed in the spontaneous emission. Therefore, wearing only the spontaneous emission for cooling at ( averaging over many absorption-emission processes). If six pairs opposite rays used, as shown above, so can the atoms slow down and cool off in all directions in space. The following figure illustrates the braking process schematically:

Catch

If one were to use only this principle, one would obtain, although cooled atoms, but they would diffuse out of the field of cooling, since there is only one speed-dependent, but not location-dependent force. Adds one now added a the distance from the cooling region linearly increasing magnetic field (eg, an anti- Helmholtz arrangement of coils ), the result is due to the Zeeman splitting of atomic states, which is proportional to the magnetic field, a restoring force by the laser. Due to the selection rules of the Zeeman effect, it is now necessary to irradiate circularly polarized light, otherwise no interaction with the nuclear states would be possible.

The drawing on the right explains the process in more detail for a simple model system (F = 0 to F = 1 transition). The magnetic field leads to a position-dependent splitting of the levels in a higher, a lower and an unaltered state ( mF = ± 1.0). Since the laser is red - detuned by Δω from the resonance frequency, it comes at a particular position in resonance with the lowered or raised. The left of the cooling region, there is a point at which the laser cooling is in resonance with the lowered state ( mF = 1). This can only interact with the light helicity σ due to the selection rules. The same is true for σ - light, right from the cooling region with the ( mF = -1 ) state. So you beam from left σ - and σ from the right - a light, this results in a net force that repels the atoms in the cooling region.

The helicities translate in the laboratory in circular polarizations in such a way that a pair of counter-rotating beams each has the same polarization. This fact is somewhat confusing. It is because we consider the rotation of the light even with respect to the propagation direction of the beam ( polarization of light ) and once with respect to the quantization of the atom ( σ helicity or σ ). The quantization axis is parallel to the magnetic field at the point x = 0, it thus turns around. For the atom is only the direction of rotation relative to said axis of importance, but not the direction of the light comes. So that the rotation of the quantization results ( upon reversal of the magnetic field ) in the reference system of the atom in a change in the direction of rotation of the light.

A magneto- optical trap can only atoms which are slower than a certain maximum speed trap. For faster atoms braking is no longer sufficient by the Absorptions-/Emissionszyklen and they are slower, but not caught. This is clarified in the following graph ( according to location velocity v z ) of atoms shows the trajectories with different starting speeds. Too fast atoms are slowed down, but not caught. The gray profile is the intensity profile of the MOT laser on ( right axis! , IS is the so-called saturation intensity of the transition). The magnetic field was assumed to be linear ( which is for the center region of a MOT A., a good approximation ).

Real atoms

Real atoms usually have several states into which the excited atom can decay, although not all interact with the cooling light. To prevent the loss of such atoms decays from the trap is used, depending on the atomic species or a plurality of return pump laser, the transfer states of these competing again in the cooling process returns.