Indirect Methods to Control Population Distribution in a Large Spin   System
  We demonstrate how a large spin system ($S=7/2$) with the ground and the first excited state separated by a seven photon transition exhibit non equilibrium 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 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 life times are observed due to large number of photons required for the transition between the ground and the first excited states. 
Lingfei Zhao, Maxim Goryachev, Jeremy Bourhill, Michael E. Tobar
quant-ph cond-mat.mtrl-sci cond-mat.stat-mech