LOGICAL REVERSIBILITY IN QUANTUM MEASUREMENT - GENERAL-THEORY AND SPECIFIC EXAMPLES

A measurement process is logically reversible if the premeasurement de nsity operator of the measured system is uniquely determined from the postmeasurement density operator and the outcome of the measurement. T his paper analyzes the necessary and sufficient condition for a measur ement process to be logically reversible and discusses specific exampl es on quantum-nondemolition measurements. quantum counting, and measur ement of spin systems. It is shown that for any sharp measurement we c an construct a logically reversible measurement that continuously appr oaches the sharp measurement with a decrease in the measurement error. A general condition for a measurement process to be reversed by anoth er with a nonzero probability of success is given, and the implication s of such physical reversibility are discussed.