Limits on inference of gravitational entanglement

Combining gravity with quantum mechanics remains one of the biggest challenges of physics. In the past years, experiments with optomechanical systems have been proposed that may give indirect clues about the quantum nature of gravity. In a recent variation of such tests [Carney et al., PRX 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 semiclassical 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 modeling of the oscillator. We also show that the nonclassicality 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 nonclassicality 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 nonclassicality assumptions constitute a significant challenge in their own right.

Automated summary

Combining gravity with quantum mechanics is still a major challenge in physics. Recent experiments propose a method to indirectly observe the quantum nature of gravity by gravitationally entangling an atom interferometer with a mesoscopic oscillator. However, studying semiclassical models reveals that fulfilling the nonclassicality assumptions required for this experiment is challenging, making it difficult to deduce entanglement.