Reversible Quantum Measurements on a Spin 1/2 and Measuring the State of a Single System

A measurement procedure on a spin 1/2 system is described, whose effect on the (possibly unknown) state of the system can be reversed, by means of other similar measurements, with a sizable probability of success Pʳᵉᵛ (e.g., 1/2<Pʳᵉᵛ<1, depending on the state). Repeated such measurements followed by reversal, on a single system, allow us to determine its (a priori unknown) state operator with a probability of success which is nonzero, though extremely small (because Pʳᵉᵛ<1).