Retrodiction for quantum optical communications - art. no. 022313

Given the result of a measurement on the output of a quantum optical commun ication channel, we show how to calculate a retrodictive state at the input . This state can be used by the receiver to determine the probability that any one of a given set of states was selected by the transmitter. We establ ish the remarkably simple result that retrodicting the prepared input signa l for an attenuating (amplifying) channel corresponds to predicting the mea sured output signal for an amplifying (attenuating) channel.