Stopped Light Turns a Tiny Torsion Pendulum into a Super-Sensitive Gravity Sensor

Gravity is famously the weakest of the four fundamental forces, and nowhere is that weakness more humbling than at small scales. A millimetre-sized metal ball pulls on its surroundings with a force so feeble that it is easily swamped by stray electric fields, air currents and vibrations. Yet measuring such tiny gravitational tugs matters enormously: it underpins tests of Newton’s inverse-square law at short distances, searches for exotic forces that might hint at extra dimensions or dark-energy physics, and the long-term ambition of probing whether gravity itself obeys quantum rules. Now a team at Shanghai Jiao Tong University, writing in Nature Nanotechnology, has reported a compact gravitational-force sensor that marries a classical Cavendish-style torsion pendulum to a whispering-gallery optical microcavity, using a striking phenomenon called stopped light to squeeze out unprecedented sensitivity.

The experiment is, at heart, a modern descendant of Henry Cavendish’s 1798 torsion balance, the apparatus he used to weigh the Earth. A torsion pendulum responds to a horizontal force by twisting a thin suspension fibre, and because the restoring torque of such a fibre can be made extraordinarily small, even minuscule forces produce measurable deflections. Cavendish-type balances remain among the most sensitive instruments for measuring gravity from small source masses, and they have been used to pin down the Newtonian gravitational constant and to test whether gravity behaves as expected at centimetre and sub-centimetre scales. The challenge the Shanghai-led team set out to solve is one of readout: how do you detect the nanometre-scale motion of a pendulum twisted by a gravitational pull of order 10^-16 newtons without adding noise, bulk or complexity?

Their answer is an optical microcavity coupled directly to the pendulum. The sensor uses a whispering-gallery-mode microresonator, a tiny optical resonator in which light circulates around the rim of a microsphere by continuous total internal reflection. Light is coupled into and out of the cavity through a tapered optical fibre whose tip is held roughly 100 nanometres from the microsphere surface. In this evanescent coupling regime, the amount of light that crosses the nanoscale gap depends exquisitely on its exact width. When the torsion pendulum moves, it changes the cavity-fibre separation, which in turn changes the optical transmission. The exponentially decaying evanescent field thus converts nanoscale displacements into optical signals across that roughly 100-nanometre coupling window, acting as an extremely steep mechanical-to-optical transducer.

The real innovation, however, lies in how the team manipulated the light inside the cavity. By exploiting coupled photon-phonon effects, in which the optical field interacts coherently with a mechanical acoustic mode of the resonator, they induced a regime known as Brillouin-scattering-induced transparency, an analogue of electromagnetically induced transparency familiar from atomic physics. Within this narrow spectral window, the optical dispersion becomes extremely steep, and the group velocity of light, the speed at which information-carrying pulses propagate, collapses to just 2,000 metres per second. That is roughly 150,000 times slower than light in vacuum, and it places the device firmly in the ‘stopped light’ regime that the same group has explored previously for light storage near exceptional points.

Why does slowing light help? The steep dispersion that accompanies stopped light means that a tiny change in the cavity resonance frequency, caused by a tiny displacement of the pendulum, produces a disproportionately large change in the phase and transmission of the probe light. In effect, the slow-light resonance amplifies the optical signature of small mechanical motion. This is conceptually similar to how optomechanically induced transparency and other cavity-enhanced readout schemes have boosted force sensitivity in micro- and nanomechanical resonators, but the stopped-light approach pushes the dispersion slope to an extreme, sharpening the transduction without requiring cryogenic temperatures or exotic materials. The whole readout chain operates at room temperature in a table-top apparatus.

The measured performance is impressive. The team reports a displacement sensitivity of 7.85 picometres per root hertz, meaning they can resolve pendulum motions of a few trillionths of a metre in a one-hertz measurement bandwidth. Converted through the mechanical response of the torsion pendulum, that corresponds to an acceleration sensitivity of 3.06 x 10^-16 g, where g is the standard acceleration due to Earth’s gravity. To put that in perspective, the sensor can detect accelerations roughly sixteen orders of magnitude smaller than the pull you feel standing on the ground. By periodically modulating the position of the source mass, a modulation technique that shifts the gravitational signal to a known frequency where technical noise is lower, the researchers resolved a minimum detectable change in gravitational force of 3.02 x 10^-16 newtons.

The source mass in these experiments was millimetre-scale, comparable to the masses used in the landmark 2021 Vienna experiment that first measured gravitational coupling between millimetre-sized gold spheres. That experiment demonstrated that gravity between small objects can be isolated; the new work shows how a photonic readout can be integrated with such a mechanical sensor to push sensitivity further while keeping the instrument compact. Integration has long been the sticking point for Cavendish-type balances: their optical levers and interferometric readouts are delicate, alignment-heavy and difficult to miniaturise. A microcavity readout, with light delivered through a fibre taper and the transduction happening in a micrometre-scale resonator, points toward on-chip gravitational sensing platforms.

The implications extend beyond metrology. Detecting the gravitational fields of microscopic objects is a prerequisite for microscale gravitational imaging, for constraining short-range deviations from Newtonian gravity, and for proposed tests of gravity’s quantum nature. Several theoretical proposals, including the influential 2017 Bose and Marletto-Vedral schemes, suggest that if two masses become entangled purely through their mutual gravitational interaction, gravity must be quantum-mechanical. Realising such experiments demands sensors that can resolve gravitational forces from ever-smaller masses held in ever-more-controlled quantum states. The acceleration sensitivity demonstrated here, combined with proposals for detecting single gravitons with quantum sensing and for gravitationally induced decoherence tests, suggests that stopped-light-enhanced readout could become a key enabling technology on that road.

There are, of course, practical hurdles between a table-top demonstration and a quantum-gravity laboratory. Torsion pendulums are inherently susceptible to thermal noise in the suspension fibre, seismic disturbances and Newtonian background forces from nearby objects, and the evanescent coupling gap of 100 nanometres demands nanometre-precision positioning stability. The researchers also note that the coupled photon-phonon physics that produces stopped light is the same physics that can introduce optomechanical backaction, so careful operating-point selection is needed to ensure that the dispersion enhancement is not eroded by the light’s own mechanical influence. Nonetheless, the demonstration that a room-temperature microcavity can transduce gravitational forces at the 10^-16-newton level marks a genuine advance in nanometrology.

What makes the result especially compelling is its elegance: rather than adding complexity, the team exploited a fundamental optical effect to make the sensor simultaneously simpler and more sensitive. Slow and stopped light has been celebrated for applications ranging from optical buffering to nonlinear enhancement, but using it as the readout engine of a gravitational sensor is a genuinely unexpected twist. If the approach can be scaled to smaller source masses, integrated with levitated or cryogenic mechanical systems, or combined with quantum state preparation of the pendulum itself, the humble Cavendish experiment may yet become a window into the deepest open question in physics: whether the force that holds the cosmos together plays by quantum rules at the smallest scales.

Subject of Research: Stopped-light-enhanced torsion-pendulum sensing of gravitational forces from millimetre-scale masses

Article Title: Stopped-light-enhanced gravitational force sensing

Article References: Zhu, Y., Geng, Q., Xue, B., Sun, Y., Zhang, R., Chen, X., Jiang, X., Azeem, F., Tierz, M., & Wan, W. (2026). Stopped-light-enhanced gravitational force sensing. Nature Nanotechnology. https://doi.org/10.1038/s41565-026-02298-8

Image Credits: AI Generated

DOI: 10.1038/s41565-026-02298-8

Keywords: gravity sensing, torsion pendulum, stopped light, optical microcavity, whispering gallery mode, cavity optomechanics, nanometrology, slow light, force sensitivity, quantum gravity, photon-phonon coupling, Nature Nanotechnology