Observation of Radiation Pressure Shot Noise

The quantum mechanics of position measurement of a macroscopic object is typically inaccessible due to strong environmental coupling and classical noise. Here we show that a micromechanical membrane resonator subject to an increasingly strong continuous position measurement exhibits a quantum mechanical back action force that rises in accordance with the Heisenberg uncertainty relation. For our optically-based position measurements, the specific form of the back action is that of a fluctuating radiation pressure from the Poisson distributed photons in the coherent measurement field, termed radiation pressure shot noise. We demonstrate a radiation pressure shot noise drive that is comparable in magnitude to the thermal forces in our system. Additionally, we observe a temporal correlation between the fluctuations in radiation force and position of the resonator that may also be interpreted as a quantum non-demolition measurement of the photon field.