Active laser cooling of a centimeter-scale torsional oscillator

Experimental tests of gravity's fundamental nature call for mechanical systems in the quantum regime while being sensitive to gravity. Torsion pendula, historically vital in studies of classical gravity, are ideal for extending gravitational tests into the quantum realm due to their inherently high mechanical quality factor, even when mass-loaded. Here, we demonstrate laser cooling of a centimeter-scale torsional oscillator to a temperature of 10 mK (average occupancy of 6000 phonons) starting from room temperature. This is achieved by optical radiation pressure forces conditioned on a quantum-noise-limited optical measurement of the torsional mode with an imprecision 9.8 dB below its peak zero-point motion. The measurement sensitivity is the result of a novel `mirrored' optical lever that passively rejects extraneous spatial-mode noise by 60 dB. The high mechanical quality ($1.4\times 10^7$) and quantum-noise-limited measurement imprecision demonstrate the necessary ingredients for realizing the quantum ground state of torsional motion -- a pre-requisite for mechanical tests of gravity's alleged quantum nature.