Optical interferometric readout of a magnetically levitated superconducting microsphere

We probe the motion of a 6-μ⁢g magnetically levitated superconducting microsphere using optical interferometry at 3 K, achieving a resolution of the order of 1 nm/√Hz, and use the measured signal to feedback-cool its motion. The resolution exceeds the shot-noise limit of 11 pm/√Hz primarily due to technical noise arising from the roughness of the particle, in combination with its free rotation. These measurements demonstrate that the combination of interferometric optical readout and superconducting magnetic levitation, together with established techniques of cavity optomechanics, provides all the ingredients required for quantum physics experiments with microgram masses.