A USEFUL MEASURE OF QUANTUM STATE IMPURITY

We employ M=[(Trrho)2-Trrho2]/(Trrho)2 as a quantitative measure of st ate impurity, i.e. the deviation of a time-dependent quantum state des cribed by density matrix rho, from a pure state. Two examples, an opti cally prepared molecular state and a quantum kicked rotor subjected to Gaussian amplitude noise are considered. In the first case M quantifi es the relationship between state impurity and properties of the molec ule and of the light. In the second case increasing M quantitatively t racks the loss of coherence due to external noise.