Stopped-light-enhanced gravitational force sensing

As the weakest fundamental force, gravity yields only small accelerations from millimetre-scale masses, making them difficult to detect. Detecting the gravitational fields of microscopic objects is important for microscale sensing, imaging or studies of the impact of the quantum state of an object on gravity. Cavendish-type torsion balances can sensitively measure gravity from small source masses, but their integration with micro- and nanophotonic readout remains challenging. Here we show gravitational-force sensing of a millimetre-sized mass using a torsion pendulum integrated with optical microcavity readout. A coupled photon-phonon resonance induces stopped light behaviour, reducing the group velocity to 2,000 m s^-1 and producing steep optical dispersion. The exponentially decaying evanescent field of the microcavity converts nanoscale displacements over an ~100 nm coupling length into optical signals, thereby increasing displacement sensitivity. The sensor reaches a displacement sensitivity of 7.85 pm/ $\sqrt{Hz}$ , corresponding to an acceleration sensitivity of 3.06 × 10^-16 g. By periodically modulating the mass position, we resolve a minimum detectable gravitational-force change of 3.02 × 10^-16 N. This compact, sensitive scheme supports microscopic gravitational sensing and may allow for studies of gravity's quantum nature.