Laser cooling a 1-milligram torsional pendulum to 240 microkelvins

Milligram-scale mechanical oscillators hold potential for testing fundamental physical phenomena on various fronts, including laboratory-scale effects at the interface of quantum mechanics and gravity. Nevertheless, experimental realizations supporting this potential have thus far been lacking. Here, we present a new level of control over a 1-milligram torsional pendulum, demonstrating laser feedback cooling of its torsional motion from room temperature to 240 microkelvins, establishing the coldest mechanical motion in the microgram-to-kilogram mass window -- a factor of 25 beyond prior art. The developed system shows a unique torque sensing capability, reaching 1.2×10⁻¹⁸ N·m/√Hz thermal-noise-limited sensitivity -- a factor of 10 below the state of the art for the milligram scale. With readily feasible improvements in the pendulum suspension, a further 100-fold reduction in both achievable temperatures and torque sensitivities is foreseen, pushing towards operation in the quantum regime at such large mass scales.