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I've been reading the seminal Zurek papers on decoherence but there is one (crucial) point on which I am confused. I understand the mathematical demonstrations that over very short timescales the superpositions of states represented as off-diagonal terms in the density matrix can be shown to go to zero over very short timescales due to interaction of the apparatus/system with the environment, leaving a diagonal density matrix. However, why exactly does a diagonal density matrix mean that we can never measure a superposition of states?

Thanks for any insight!

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# Decoherence and the Density Matrix

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