Measuring Unrecorded Measurement

Projective (Von Neumann) Measurement of an operator (i.e. a dynamical variable) selected from a prescribed set of operators is termed unrecorded measurement (URM) when both the selected operator and the measurement outcome are unknown, i.e. "lost". Within classical physics a URM is completely inconsequential: the state is unaffected by measurement. Within quantum physics a measurement leaves a mark. The present study provides protocols that allow retrieval of some of the data lost in a URM.