Feedback cooling the fundamental torsional mechanical mode of a tapered optical fiber to 30 mK

We experimentally study the fundamental torsional mechanical mode of the nanofiber waist of a tapered optical fiber. We find that this oscillator with a resonance frequency off = 161.7 kHz features a quality factor of up to Q = 107 and a Q-frequency product of Qf = 1 THz. The polarization fluctuations of a transmitted laser field serve as a probe for the torsional motion. We damp the oscillator's thermal motion from room temperature to 28(7) mK by means of active feedback. Our results might enable new types of fiber-based sensors and lay the foundation for a promising hybrid quantum optomechanical platform.