Information, fidelity, and reversibility in photodetection processes

Four types of photon counters are discussed in terms of information, fidelity, and physical reversibility: conventional photon counter, quantum counter, and their quantum nondemolition (QND) versions. It is shown that when a photon field to be measured is in an arbitrary superposition of vacuum and one-photon states, the quantum counter is the most reversible, the QND version of conventional photon counter provides the most information, and the QND version of quantum counter causes the smallest state change. Our results suggest that the physical reversibility of a counter tends to decrease the amount of information obtained by the counter.