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arXiv:2508.11731 · Physical Review Applied

Optical interferometric readout of a magnetically levitated superconducting microsphere

Jannek J. Hansen, Stefan Minniberger, Dominik Ilk, Peter Asenbaum, Gerard T. Higgins, Rhys G. Povey, Philip Schmidt, Joachim Hofer, Remi Claessen, Markus Aspelmeyer, Michael Trupke

Published Mar 27, 2026• 1 citation
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Abstract

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.

DOI
10.1103/ljxp-3f66

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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.
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We present a cavity-electromechanical system comprising a superconducting quantum interference device which is embedded in a microwave resonator and coupled via a pickup loop to a 6 - μ g magnetically levitated superconducting sphere. The motion of the sphere in the magnetic trap induces a frequency shift in the SQUID-cavity system. We use microwave spectroscopy to characterize the system, and we demonstrate that the electromechanical interaction is tunable. The measured displacement sensitivity of 10 − 7 m / Hz defines a path towards ground-state cooling of levitated particles with Planck-scale masses at millikelvin environment temperatures. Published by the American Physical Society 2024
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Magnetically levitated superconducting microparticles provide a promising platform for fundamental quantum experiments and quantum-limited sensing. In the Meissner state, the levitated particle is trapped without mechanical clamping, allowing its center-of-mass motion to exhibit ultralow dissipation. The particle's displacement can be read out dispersively via magnetic flux coupling to superconducting resonators. A challenge for magnetomechanical coupling is to realize a flux-tunable microwave circuit that provides strong flux-to-frequency transduction, low dissipation, and a sufficiently large linear operating range for readout and cooling. This thesis develops and characterizes a chip-based superconducting platform for realizing this transduction.Chip-based levitation of a superconducting microparticle is demonstrated at 40 mK. Passive cryogenic vibration isolation is developed to increase the mechanical quality factors of the levitated particle. For coupling to a microwave circuit, a superconducting flip-chip process based on indium microspheres was developed. The need for Au passivation of Nb or NbN under-bump metallization is demonstrated, and indium-based superconducting interconnects are shown to carry ampere-scale currents. Flux-tunable resonators with 100-200 µm SQUID loops and integrated input coils are designed, fabricated, and characterized. A peak flux responsivity of 20 GHz/Φ₀ is achieved, with one flux quantum modulation requiring only 10-20 µA of input-coil current. Two integrated flux-biasing architectures are developed with flip-chip and on-chip input coils, achieving a total flux transfer efficiency of 1.6%. Junction asymmetry is shown to suppress branch-switching behavior associated with finite SQUID screening.Together, these results establish the superconducting subsystems required for dispersive microwave readout of the center-of-mass motion of a levitated microparticle. Such a magnetomechanical coupling provides a route toward cooling the center-of-mass motion to the ground state, paving the way for the preparation of nonclassical motional states. More broadly, such systems are well-suited for quantum-limited force and acceleration sensing, as well as for probing quantum physics in a previously unexplored mass regime.