  All compositions of a mixed-state density operator are equivalent for the
prediction of the probabilities of future outcomes of measurements. For
retrodiction, however, this is not the case. The retrodictive formalism of
quantum mechanics provides a criterion for deciding that some compositions are
fictional. Fictional compositions do not contain preparation device operators,
that is operators corresponding to states that could have been prepared. We
apply this to Molmer's controversial conjecture that optical coherences in
laser light are a fiction and find agreement with his conjecture. We generalise
Molmer's derivation of the interference between two lasers to avoid the use of
any fictional states. We also examine another possible method for
discriminating between conerent states and photon number states in laser light
and find that it does not work, with the equivalence for prediction saved by
entanglement.
