  Two measures of fidelity are proposed for postselecting devices, the
retrodictive conditional probability that the state in the measurement arm is
the one indicated by the detectors, and the probability that the device
produces the state that it would produce if working perfectly. The first is the
natural quantity that one wishes to maximise to improve the device operation.
The second corresponds more closely with the accurate operation of the device
than the more usual overlap-based fidelity. The results are particularly
applicable to the types of state preparation and logic gate operation in linear
optical quantum computing.

Quantum Physics