Quantum nature of laser light
  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. 
David T. Pegg and John Jeffers