Resolved sideband cooling is a laser cooling technique to cool a ion in a trap (for example Penning trap) to the ground state of motion. The ion is first cooled down using the Doppler cooling technique. However the motion of the ion is still very classical. Subsequently the resovled sideband cooling is used to cool the ion beyond the Doppler temperature.

A trapped ion can be treated to a good approximation as a quantum mechanical harmonic oscillator. If the spontaneous decay rate is much smaller than the vibrational frequency of the ion in the trap, the energy levels of the system can be resolved as consisting of internal levels each corresponding to a ladder of vibrational states.

Suppose a two-level atom whose ground state is shown by g and excited state by e. Efficient laser cooling occurs when the frequency of the laser beam is tuned to the red sideband i.e.

ω = ω₀ - ω_z.,

where ω₀ is the internal atomic transition frequency and ω_z is the harmonic oscillation frequency of the atom. In this case the atom undergoes the transition

,

where represents the state of an ion whose internal atomic state is a and the motional state is m.

Subsequent spontaneous emission occurs predominantly at the carrier frequency if the recoil energy of the atom is negligible compared with the vibrational quantum energy i.e.

The average effect of this mechanism is cooling the ion by one vibrational energy level. When these steps are repeated a sufficient number of times is reached with a high probability.

See also

• Laser cooling