  According to quantum theory, measurement and backaction are inextricably
linked. In optical position measurements, this backaction is known as radiation
pressure shot noise. In analogy, a measurement of the orientation of a
mechanical rotor must disturb its angular momentum by radiation torque shot
noise. In this work, we observe the shot-noise torque fluctuations arising in a
measurement of the angular orientation of an optically levitated nanodumbbell.
We feedback cool the dumbbell's rotational motion and investigate its reheating
behavior when released from feedback. In high vacuum, the heating rate due to
radiation torque shot noise dominates over the thermal and technical heating
rates in the system.
