Experimental Test of Quantum Gravity Induced Non-locality Using a Liquid Helium Filled Cavity*

Mesoscopic mechanical devices in the quantum regime can play a key role in quantum sensing, including in tests of quantum gravity and in dark matter searches. A number of these applications require the generation of high-amplitude quantum coherent states in large-mass systems. Here we present measurements of the state purity of a strongly-driven acoustic mode of a nanogram-scale superfluid resonator. We use photon counting techniques to measure the mode’s phonon-phonon correlations and observe that these correlations evolve from bunched statistics for very low drives to Poissonian statistics for strong drives. Quantitative analysis of this data shows that the state maintains a roughly constant variance (~2 phonons) even while being displaced by up to 10⁵ phonons. We use these measurements (together with the analysis proposed in Ref. [1]) to constrain the quantum gravity non-locality length down to 10^-18 m.