Laser Cooling of an Optomechanical Crystal Resonator to Its Quantum Ground State of Motion

Abstract

Quantum mechanics continues to intrigue us with bizarre predictions that seemingly run counter to our everyday classical intuition. Superposition, zero-point motion, entanglement, and inescapable bounds on measurement precision are just a few purely quantum mechanical effects that come to mind. The promise of observing such effects in mesoscale mechanical resonators some orders of magnitude larger than the systems these effects had once been confined to, has resulted in surging interest in the field of cavity electro- and optomechanics. In these systems, the strong interaction of light and matter allows radiation pressure forces to provide significant damping to the mechanical motion, and serves as a means to mitigate the quantum-destroying, decohering effects of the pervasive thermal bath. However, for this backaction cooling to reduce the phonon occupation of a mechanical mode below unity, the confluence of the device and experimental setup must conform to a very strict set of conditions characterized by high optical and mechanical cavity quality factors, low optical absorption, low drive noise, and sufficiently sensitive detection. In this work, we describe the first optomechanical device and all-optical experimental setup to simultaneously satisfy these conditions, realizing the quantum ground state cooling of a 3.7 GHz mechanical mode (with a final phonon occupation of 0.85 +/- 0.08) in a picogram and micron-scale patterned nanobeam structure from a bath temperature of approximately 20 K. In context, subunity occupation of a mechanical mode in a similar-sized object had previously only been achieved by electromechanical devices operating in millikelvin dilution refrigerator environments. We also discuss the numerical simulation efforts involved in designing and optimizing these novel, coupled optical and mechanical resonators, and the fabrication procedure to realize them in silicon microchips. We recognize that this cooling result represents only an initial step toward the complete optical control of mesoscale mechanical oscillators in the quantum regime. To this end, we summarize an experiment we performed to detect the quantum zero-point motion of a nanobeam via scattering sideband asymmetry. We further show work in improving the optomechanical coupling and quality factors of these devices, as well as devising more efficient coupling schemes to improve measurement sensitivity.