Demonstrating levitation within a microwave cavity

TL;DR

This paper demonstrates the first levitation of a millimeter-scale magnet within a microwave cavity, enabling force sensing and potential quantum system coupling at cryogenic temperatures.

Contribution

It reports the first experimental realization of magnet levitation inside a microwave cavity, with detailed analysis of levitation behavior and temperature dependence.

Findings

Achieved levitation heights of 1-1.8 mm.

Observed increased levitation temperature and height with stronger magnets.

Confirmed levitation through resonance frequency shifts at cryogenic temperatures.

Abstract

Levitated systems are desirable due to reduced clamping losses and reduced thermal contact. These advantageous properties have been exploited in optomechanics to achieve ultra-strong coupling between the mechanical mode and the electromagnetic mode. Such schemes provide an opportunity for the quantum manipulation of a macroscopic system. In this letter, we report the first successful experiments with a levitated millimeter-scale neodymium magnet within a centimeter-scale superconducting aluminum coaxial quarter-wave stub cavity. The magnet levitated near the top of the stub, where the electric field is concentrated, perturbs the electric field distribution allowing for small perturbations in the magnet's position to be detected through shifts in the resonance frequency. Resonance spectra are collected via a vector network analyzer (VNA) between temperatures of 5 K and 50 mK revealing movement of the magnet inside of the cavity. Room temperature measurements and finite element calculations are done to calculate the shift in frequency for various positions of the magnet, and an experimentally measured 100 MHz upshift when transitioning into a superconducting state confirms levitation with remanences up to 140 times stronger than the critical field of the aluminum. We achieve levitation heights of 1 - 1.8 mm. We investigate the dependence of levitation height and levitation temperature on the strength of the magnet and, surprisingly, we observe that the levitation temperature and height both increase with permanent magnet strength. Our work describes a novel macroscopic mechanical system capable of sensing and transducing forces, thus allowing for the coupling of disparate classical and quantum systems.