Indirect methods to control population distribution in a large spin system

Abstract

We demonstrate how a large spin system ( $S=7/2$ ) with the ground and first excited state separated by a seven-photon transition exhibits nonequilibrium thermodynamic properties and how the population distribution may be manipulated using coupling between energy levels. The first method involves non-adiabatic passage through an avoided level crossing controlled with an external DC magnetic field and the resulting Landau–Zener transition. The second method is based on external cavity pumping to a higher energy state hybridised with another state that is two single-photon transitions away from the ground state. The results are confirmed experimentally with a Gd ^3+ impurity ion ensemble in a YVO _4 crystal cooled to 20 mK, which also acts as a microwave photonic whispering gallery mode resonator. Extremely long lifetimes are observed due to the large number of photons required for the transition between the ground and first excited states.

Keywords

• spins in solids
• large spin system
• whispering gallery mode
• Landau–Zener transition