Limits on inference of gravitational entanglement

TL;DR

This paper critically examines proposed experiments to detect gravitationally induced quantum entanglement, showing that classical models can mimic the expected signatures, thus challenging the interpretation of such experiments as evidence of quantum gravity.

Contribution

It demonstrates that the key signatures of gravitational entanglement can be reproduced by semi-classical models, questioning the reliability of current experimental approaches.

Findings

Classical models can produce visibility collapses and revivals.

Non-classicality of the oscillator is limited unless near ground state.

Detecting entanglement requires verifying non-classicality, which is highly challenging.

Abstract

Combining gravity with quantum mechanics remains one of the biggest challenges of physics. In the past years, experiments with opto-mechanical systems have been proposed that may give indirect clues about the quantum nature of gravity. In a recent variation of such tests [D. Carney et al., Phys.Rev.X Quantum 2, 030330 (2021)], the authors propose to gravitationally entangle an atom interferometer with a mesoscopic oscillator. The interaction results in periodic drops and revivals of the interferometeric visibility, which under specific assumptions indicate the gravitational generation of entanglement. Here we study semi-classical models of the atom interferometer that can reproduce the same effect. We show that the core signature -- periodic collapses and revivals of the visibility -- can appear if the atom is subject to a random unitary channel, including the case where the oscillator is fully classical and situations even without explicit modelling of the oscillator. We also show that the non-classicality of the oscillator vanishes unless the system is very close to its ground state, and even when the system is in the ground state, the non-classicality is limited by the coupling strength. Our results thus indicate that deducing entanglement from the proposed experiment is very challenging, since fulfilling and verifying the non-classicality assumptions is a significant challenge on its own right.