By shining laser light through a nanomechanical beam, we measure the beams thermally driven vibrations and perturb its motion with optical forces at a level dictated by the Heisenberg measurement-disturbance uncertainty relation. Such quantum effects are typically difficult to observe at room temperature where the motion driven by optical quantum intensity fluctuations is many orders of magnitude smaller than the thermal motion. We demonstrate a cross correlation technique to distinguish optically-driven motion from thermally driven motion, observing this quantum backaction signature up to room temperature. While it is often difficult to absolute calibrate optical detection, we use the scale of the quantum correlations, which is determined by fundamental constants, to gauge the size of thermal motion demonstrating a path towards absolute thermometry with quantum-mechanically-calibrated ticks.
cavity optomechanics, quantum measurement backaction, Brownian motion, thermometry