The ground-state cooling and quantum control of levitated nanoparticles have become attainable, which makes the enhanced quantum sensing based on the large mass advantage of mesoscopic quantum systems accessible. Here, we propose an enhanced gravity-sensing protocol for a nanodiamond levitated in a linear ion trap, which improves the sensing sensitivity and further reduces device size. By introducing gravitational acceleration into the vibrational state through transient free fall, we achieve a square-accelerated relative phase accumulation, yielding higher sensitivity than the linear accumulation process. The sensitivity reaches the standard quantum limit for a nanodiamond containing multiple nitrogen-vacancy (N-V) centers. Besides, by using phonon-mediated one-axis twisting dynamics induced by magnetic field gradient to squeeze the initial spin state, the sensitivity can further surpass the standard quantum limit as a scaling law of $N^{-5/6}$. To fully assess the protocol performance, we further analyze the effects of decoherence and control errors, which are negligible under experimentally achievable conditions. Specifically, the sensitivity reaches $\sim 18\mathrm{\mu }\mathrm{Gal}/\sqrt{\mathrm{Hz}}$ for the nanodiamond containing only one N-V center over a free-fall distance of $\sim 20\mathrm{\mu }\mathrm{m}$, while exceeds $0.4\mathrm{\mu }\mathrm{Gal}/\sqrt{\mathrm{Hz}}$ for 100 N-V centers. Our results will further facilitate research on compact and high-sensitivity gravimeters.

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• Quantum harmonic oscillator
• Quantum metrology
• Quantum sensing
• Nano-oscillators