Cooling the optical-spin driven limit cycle oscillations of a levitated gyroscope

TL;DR

This paper demonstrates how to suppress non-conservative optical forces on spinning birefringent microspheres, enabling stable, cooled, and coherent nano-scale limit cycle oscillations in optical traps, with potential for quantum applications.

Contribution

It reveals the suppression of azimuthal forces on birefringent particles and introduces parametric feedback for dissipation, enabling stable, cooled, and coherent limit cycles in optical trapping.

Findings

Suppressed azimuthal forces in birefringent microspheres stabilize rotation.

Parametric feedback creates dissipation without thermal noise.

Achieved milliKelvin effective temperatures for limit cycles.

Abstract

The non-conservative, azimuthal forces associated with inhomogeneous optical-spin angular momentum play a critical role in optical trapping. Intriguingly, birefringent microspheres can be stably levitated and rapidly rotated in circularly polarized optical traps in ultra-high vacuum whereas isotropic spheres are typically destabilized and expelled, even at relatively modest pressures. Here we show that the resolution of this apparent key paradox rests in the form of the orientationally averaged, effective forces acting on the spinning birefringent particle. In particular, the effective azimuthal component is heavily suppressed and highly non-linear. As a consequence, non-conservative effects are strongly, if imperfectly, inhibited. Their influence is apparent only at very low pressures where we observe the formation of noisy, nano-scale limit cycles or orbits. Finally, we show how parametric feedback can synthesize a form of dissipation, necessary to preserve limit cycle oscillation, without introducing additional thermal fluctuations. This allows the preparation of highly coherent, self-sustained oscillations with effective temperatures on the order of a milliKelvin. The tailoring of azimuthal spin forces through the material structure of a spinning, non-spherical particle opens up new opportunities for the design of ultra stable optical rotors. In addition, we have shown that the unique profile of the azimuthal force, featured in this work, allows for the formation of nano-scale limit cycles that can be stabilized and cooled. In principle, this approach could enable the cooling of limit cycles into the quantum regime, allowing for experimental realisation of quantum synchronization, or alternative ways of entangling mesoscopic bodies.