Active radiation pressure cooling of a cm-scale torsion pendulum

We demonstrate a laser cooling of a centimeter-scale torsional oscillator down to a temperature of 10 mK (6000 phonons) starting from room temperature. This cooling is enabled by applying optical radiation pressure forces conditioned on quantum-noise-limited angular displacement measurements, achieving an imprecision level 13 dB below the on-resonance standard quantum limit (SQL). The enhanced precision is facilitated by a novel `mirrored’ optical lever design, which effectively suppresses spatial-mode noise by 60 dB. The torsional oscillator’s high quality factor over 10⁷ and sub-SQL measurement imprecision provide a key pathway toward reaching the quantum ground state of torsional motion, essential for probing potential quantum aspects of gravity.