Minimal sets of dequantizers and quantizers for finite-dimensional   quantum systems
  The problem of finding and characterizing minimal sets of dequantizers and quantizers applied in the mapping of operators onto functions is considered, for finite-dimensional quantum systems. The general properties of such sets are determined. An explicit description of all the minimum self-dual sets of dequantizers and quantizers for a qubit system is derived. The connection between some known sets of dequantizers and quantizers and the derived formulae is presented. 
P. Adam and V. A. Andreev and A. Isar and M. A. Man'ko and V. I.   Man'ko
quant-ph