A USEFUL MEASURE OF QUANTUM STATE IMPURITY

We employ M=[(Trrho)2-Trrho2]/(Trrho)2 as a quantitative measure of state impurity, i.e. the deviation of a time-dependent quantum state described by density matrix rho, from a pure state. Two examples, an optically prepared molecular state and a quantum kicked rotor subjected to Gaussian amplitude noise are considered. In the first case M quantifies the relationship between state impurity and properties of the molecule and of the light. In the second case increasing M quantitatively tracks the loss of coherence due to external noise.