stevendaryl said:
in the time period between Alice's measurement and the time when she receives confirmation from Bob, does Alice not have a definite result? She's in some kind of mixed, or superposed state?
According to Bohr (cited in
post #197), one has to consider the experiment as a whole to get a consistent quantum interpretation in the most orthodox sense. Thus the answer depends on how one dissects the universe into system and environment, just as in the old analysis about
Wigner's[/PLAIN] friend.
To avoid problems of Schroedinger cat type, we may assume that Alice is simply modeled as the pair of (pionter setting,color of light) in Alice's device, and similar for Bob. Yvonne is modeled in the Cartesian product by a state in the tensor product.
From the perspective of Alice, the experiment is concluded when she gets her result, and Bob's result (which she is not observing but only inferring) is not part of the setting. As a consequence, the system is in a definite state as far as Alice is concerned and Bob is in a superposition of possible pairs (pointer setting, light color). Unless Alice assumes that Bob could keep any prior agreements and uses a reduced system description that breaks Bob's superposition.
On the other hand, from the perspective of Yvonne (the coincidence counter), the experiment is concluded only when she gets her coincident result, and before that both Alice and Bob are in a superposition as obtained from the unitary dynamics prepared by Norbert.
Thus
what is definite depends on which problem description is being employed - as in any stochastic description of a system.
Indeed, this is in many ways analogous (though different in detail) to what one finds in the interpretation of a classical experiment involving throwing a sequence of labelled dice. We may consider the experiment to be performed by Alice who hides the dice thrown under a piece of cloth; later Yvonne comes and lifts the cloth slowly so that one number after the other appears.
To Alice, all dice are known and the system is in a pure state with definite outcomes. To Yvonne, no definite outcome exists initially, and her system is in a mixture of all possible sequences. As she lifts the veil from the first die, her system collapses into an eigenstate of the first die, and only a mixture of the sequences with fixed first entry results, etc. until at the end, when the veil is completely removed, her system state is collapsed to a pure state with the same definite outvcomes as that known by Alice long before.