Constraints on probing quantum coherence to infer gravitational entanglement

TL;DR

This paper critically examines a proposed quantum gravity experiment, revealing that classical models can mimic quantum signatures, and argues that current protocols are only effective under near-pure quantum states, highlighting the need for refined testing methods.

Contribution

The study demonstrates that semi-classical models can replicate experimental signatures of quantum entanglement in gravity tests, challenging the interpretation of such experiments.

Findings

Semi-classical models produce similar signatures as quantum entanglement.

Entanglement is not the source of coherence revivals in the tested regime.

Current protocols are only effective when the oscillator is nearly in a pure quantum state.

Abstract

Finding a feasible scheme for testing the quantum mechanical nature of the gravitational interaction has been attracting an increasing level of attention. Gravity mediated entanglement generation so far appears to be the key ingredient for a potential experiment. In a recent proposal [D. Carney et al., Phys. Rev. X Quantum 2, 030330 (2021)] combining an atom interferometer with a low-frequency mechanical oscillator, a coherence revival test is proposed for verifying this entanglement generation. With measurements performed only on the atoms, this protocol bypasses the need for correlation measurements. Here we explore formulations of such a protocol, and specifically find that in the envisioned regime of operation with high thermal excitation, semi-classical models, where there is no concept of entanglement, also give the same experimental signatures. We elucidate in a fully quantum mechanical calculation that entanglement is not the source of the revivals in the relevant parameter regime. We argue that, in its current form, the suggested test is only relevant if the oscillator is nearly in a pure quantum state, and in this regime the effects are too small to be measurable. We further discuss potential open ends. The results highlight the importance and subtleties of explicitly considering how the quantum case differs from the classical expectations when testing for the quantum mechanical nature of a physical system.