Feedback cooling of an insulating high-Q diamagnetically levitated plate

Levitated systems in vacuum have many potential applications ranging from new types of inertial and magnetic sensors through to fundamental issues in quantum science, the generation of massive Schrödinger cats, and the connections between gravity and quantum physics. In this work, we demonstrate the passive, diamagnetic levitation of a centimeter-sized massive oscillator, which is fabricated using a method that ensures that the material, though highly diamagnetic, is an electrical insulator. Electrical conductors moving in a magnetic field experience eddy damping—which can severely reduce their motional quality factor. By chemically coating a powder of microscopic graphite beads with silica and embedding the coated powder in high-vacuum compatible wax, we form a centimeter-sized thin square plate which magnetically levitates over a checkerboard magnet array. The insulating coating reduces eddy damping by almost an order of magnitude compared to uncoated graphite with the same particle size. These plates exhibit a different equilibrium orientation from pyrolytic graphite due to their isotropic magnetic susceptibility. We measure the motional quality factor to be Q ∼ 1.58 × 10⁵ for an approximately centimeter-sized composite resonator with a mean particle size of 12 μm. Furthermore, we apply delayed feedback to cool the vertical motion of frequency ∼ 19 Hz and achieve center-of-mass temperature decrease by three orders of magnitude.