Phase states for a three-level atom interacting with quantum fields
  We introduce phase operators associated with the algebra su(3), which is the appropriate tool to describe three-level systems. The rather unusual properties of this phase are caused by the small dimension of the system and are explored in detail. When a three-level atom interacts with a quantum field in a cavity, a polynomial deformation of this algebra emerges in a natural way. We also introduce a polar decomposition of the atom-field relative amplitudes that leads to a Hermitian relative-phase operator, whose eigenstates correctly describe the corresponding phase properties. We claim that this is the natural variable to deal with quantum interference effects in atom-field interactions. We find the probability distribution for this variable and study its time evolution in some special cases. 
A. B. Klimov, L. L. Sanchez-Soto, J. Delgado, and E. C. Yustas