This paper about Quantum Darwinism https://arxiv.org/pdf/0903.5082v1.pdf starts with statement:(adsbygoogle = window.adsbygoogle || []).push({});

"The quantum principle of superposition implies that any combination of quantum states is also a legal state. This seems to be in conflict with everyday reality: States we encounter are localized. Classical objects can be either here or there, but never both here and there. Yet, the principle of superposition says that localization should be a rare exception and not a rule for quantum systems."

This sums up nicely motivation behind decoherence approach to measurement problem.

But there is no reference for these statements so I would like to test my suspicions that this is just QM folklore rather than solid predictions of QM.

First statement: "The quantum principle of superposition implies that any combination of quantum states is also a legal state."

Isn't it required that both states are described using the same Hamiltonian? So that this statement might not be true in general?

Second: "Classical objects can be either here or there, but never both here and there." implying that quantum objects can be both here and there.

As I see this comes from explanation of double slit experiment that photon (or other quantum particle, say buckyball) goes by both paths and interferes with itself but not with other particles in the beam.

This interpretation of double slit experiment is certainly falsified by experiments that create interference between beams of two phase locked lasers.

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# B Is it a myth that serves as motivation for decoherence?

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