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 the retrieval of some of the data lost in a URM. The study is shown as supportive of viewing quantum measurement as made up of both classical-like and pure quantum components.