1-milligram torsional pendulum for experiments at the quantum-gravity interface

Abstract: Probing the possibility of entanglement generation through gravity offers a path to tackle the question of whether gravitational fields possess a quantum mechanical nature. A potential realization necessitates systems with low-frequency dynamics at an optimal mass scale, for which the microgram-to-milligram range is a strong contender. Here, after refining a figure-of-merit for the problem, we present a 1-milligram torsional pendulum operating at 18 Hz. We demonstrate laser cooling its motion from room temperature to 240~microkelvins, surpassing by over 20-fold the coldest motions attained for oscillators ranging from micrograms to kilograms. The cooling boosts the pendulum's feeble quantum coherence length, achieving the highest quantum-gravity figure-of-merit benchmarked for a table-top experiment, ranking second only to LIGO. The current approach shows a large improvement potential compared to many systems, positioning milligram-scale torsional pendulums as a prominent platform to pursue for exploring quantum aspects of gravity.