Towards observation of quantum optomechanical correlations

Quantum mechanics sets a lower limit to the precision of continuous position measurements. In an interferometric measurement, the unavoidable backaction results from the radiation pressure force exerted by the light beam on the mirrors: random position fluctuations correlated with the quantum intensity fluctuations of the light field appear as a consequence of this optomechanical coupling. Interestingly, quantum noises are expected to limit the sensitivity of the next generation of gravitational-wave interferometers over a large frequency band. Moreover, in the last decade, a number of groups have observed the effects of radiation pressure on a wide variety of micro and nanomechanical systems. These systems are now entering a regime where quantum mechanical effects of the optomechanical coupling become observable. In spite of these efforts, the effects of quantum radiation pressure force still remain elusive.In our experiment, we use two laser beams injected into the same optomechanical cavity to observe the quantum correlations between the light intensity fluctuations and the mirror motion. An intense pump beam drives the mirror into motion while ultra-sensitive position readout is performed by measuring the phase of a weak probe beam. This setup allows to recover the small effect of the optomechanical correlations by time averaging, even when the quantum radiation pressure noise (spectral density) is much smaller than the brownian noise of the mirror (spectral density). We have already observed classical correlations in a proof-of-principle experiment[1], and we are currently working on an improved version of the setup to observe quantum optomechanical correlations. With a dedicated cryogenic setup and an improved, low-mass, resonator[2], we hope to reach the regime where quantum radiation-pressure noise overcomes thermal fluctuations. Entering this regime of “quantum optomechanics” would enable a new type of quantum optics experiments, where the - echanical system itself is used as a nonlinear optical element.