Re-assessment of the state of Schroedinger's cat, final version

TL;DR

This paper rigorously proves that Schrödinger's cat is not in a superposition but in a mixed state, clarifying the measurement process and coherence transfer within quantum mechanics without external collapse mechanisms.

Contribution

It provides a rigorous proof that the cat is in a mixed state and uses two-photon interferometry experiments to clarify the nature of measurement superpositions.

Findings

The cat is in a locally mixed state, not a superposition.

Measurement transfers coherence to correlations, not to the cat or nucleus.

Explains collapse within unitary dynamics without external entities.

Abstract

The quantum state of Schroedinger's cat is usually incorrectly described as a superposition of "dead" and "alive," despite an argument by Rinner and Werner that, locally, the cat should be considered to be in a mixture of non-superposed states. Here, it is rigorously proven that the cat is not in a superposition. This is central to the measurement problem. Nonlocal two-photon interferometry experiments throw further light on the measurement state by probing the effect of a variable phase factor inserted between its superposed terms. These experiments demonstrate that both subsystems really are in locally mixed states rather than superpositions, and they tell us what the measurement state superposition actually superposes. They show that measurement transfers the coherence in Schroedinger's nuclear superposition neither to the cat nor to the nucleus, but only to the correlations between them. This explains the collapse process--but not its subsequent irreversible dissipation--within the context of unitary dynamics with no need for external entities such as the environment, a human mind, other worlds, or collapse mechanisms.