Characterizing quantum properties of a measurement apparatus: insights from the retrodictive approach.

Using the retrodictive approach of quantum physics, we show that the state retrodicted from the response of a measurement apparatus is a convenient tool to fully characterize its quantum properties. We translate in terms of this state some interesting aspects of the quantum behavior of a detector, such as the nonclassicality or the non-gaussian character of its measurements. We also introduce estimators--the projectivity, the ideality, the fidelity, or the detectivity of measurements performed by the apparatus--which directly follow from the retrodictive approach. Beyond their fundamental significance for describing general quantum measurements, these properties are crucial in several protocols, in particular, in the conditional preparation of nonclassical states of light or in measurement-driven quantum information processing.