# Entanglement and objectivity in pure dephasing models

## TL;DR

This paper explores how the emergence of objectivity in quantum systems is fundamentally linked to entanglement with observed environments, showing that entanglement is necessary for objective states to form.

## Contribution

It demonstrates that entanglement with observed environments is essential for objectivity, clarifying the role of entanglement in the quantum-to-classical transition.

## Key findings

- Entanglement with unobserved environments is irrelevant for objectivity.
- Entanglement with observed environments is crucial for the emergence of objective states.
- Orthogonality of environmental states arises from strong entanglement.

## Abstract

We study the relation between the emergence of objectivity and qubit-environment entanglement generation. We find that although entanglement with the unobserved environments is irrelevant (since sufficiently strong decoherence can occur regardless), entanglement with the observed environments is crucial. In fact, the appearance of an objective qubit-observed-environment state is strictly impossible if their joint evolution does not lead to entanglement. Furthermore, if a single observer has access to a single environment (no macrofractions) then the required orthogonality of the observed environmental states comes only as a consequence of the system-enviornment state becoming strongly entangled (maximally entangled for the given initial occupation of the qubit if the environmental state is initially pure).