Quantum back-action in recent cavity-optomechanics experiments

Quantum limits to precision measurement of an objectʼs displacement, and correspondingly the forces acting on the object, have been well studied since the 1970ʼs [1]. In the case of laser interferometers, such as those developed to detect gravitational waves, quantum fluctuations of the probe laser field set the level of imprecision for displacement sensitivity and also give rise to quantum back-action on the mechanical object being measured via radiation pressure shot noise. This part of the story, as well as back-action evading or quantum nondemolition measurement schemes, has been covered extensively theoretically. Only recently have experiments measured the effects of quantum back-action in the context of cavity- optomechanics [2-6]. I will describe two of these experiments performed at Caltech. The first involves measurement of the relative amplitude of the Stokes and anti- Stokes motional sidebands created on a probe laser field by a mechanical resonator near its quantum ground-state of motion. An asymmetry in the generated motional sidebands, as has been utilized in experiments with trapped ions and atoms, provides a self-calibrated means of measuring the mechanical oscillatorʼs quantum occupancy. An alternative view of such experiments [5], one from the perspective of continuous position measurement of the mechanical oscillator, provides an interesting twist in interpreting the source of the measured sideband asymmetry. A second experiment [6] utilizes a “strong” measurement of a silicon micromechanical resonatorʼs position to generate squeezed light. Ponderomotive squeezing in this case is observed even in the presence of significant thermal noise, and prospects for utilizing such on-chip squeezed sources look promising for precision force and inertial microsensors.