Quantum phase for a system with an arbitrary discrete energy spectrum

Abstract

Positive operator valued measures representing phase observables for systems with arbitrary discrete, possibly degenerate, spectra are constructed. The general construction is presented and discussed using illustrative examples. The phase POVM shows intricate discontinuous dependence on the eigenfrequencies. Special discussion is devoted to the systems with degenerate energy spectrum, in which case the phase observable is nonunique. We present arguments that can be used to reduces this nonuniqueness.