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by kgwgk 2568 days ago
> Throwing away this information and taking an ensemble average, you'd see an exponential decay of coherence.

That is described by a density matrix but is not a pure state. The true state may be a pure state (because as you said the systems are separable after the measurement) but our description is a mixture reflecting our imperfect information (and not a superposition of the pure states corresponding to the potential outcomes).