Self-sustained oscillations of a torsional SQUID resonator induced by Lorentz-force back-action.

For the study of nanomechanical resonators, ultra-sensitive measurement techniques are crucial. However, if the measurement sensitivity approaches quantum-mechanical limits, the back-action of the detector on the resonator cannot be neglected. If the back-action is strong enough, the corresponding instability can create self-sustained oscillators in the resonator. Here we demonstrate that a torsional mechanical resonator, which contains a direct current SQUID displacement detector, leads to this effect. We find that the Lorentz-force back-action can be so large that, in combination with complex nonlinear Josephson dynamics, it generates intrinsic self-sustained oscillations. The flux quantization limit of the maximum oscillation amplitude is exploited to calibrate the displacement resolution, which is shown to be below the standard quantum limit. The suspended torsional SQUID provides an interesting platform to study on-chip laser-like physics in an electromechanical system that can be controlled by both a flux and current bias.