  A quantum measurement is logically reversible if the premeasurement density
operator of the measured system can be calculated from the postmeasurement
density operator and from the outcome of the measurement. This paper analyzes
why many quantum measurements are logically irreversible, shows how to make
them logically reversible, and discusses reversing measurement that returns the
postmeasurement state to the premeasurement state by another measurement
(physical reversibility). Reversing measurement and unitarily reversible
quantum operation are compared from the viewpoint of error correction in
quantum computation.
