Chip-based magnetic levitation of superconducting microparticles
Doctoral thesis, 2022
This thesis is about the development and demonstration of a chip-based magnetic levitation platform for um-sized superconducting particles. To this end, we have modeled, designed, and fabricated micrometer-scale superconducting particles as well as chip-based magnetic traps based on planar superconducting coils. We have detected the center-of-mass motion of the levitated particles magnetically, with integrated superconducting coils that transport the signal of the particle motion to a SQUID magnetometer. We demonstrated stable levitation of 50um diameter particles over several days at millikelvin temperatures. This high stability allowed us to thoroughly characterize the particle motion and show that our model of the magnetic trap and the detection scheme captures the nonlinear behavior of the center-of-mass motion. These nonlinearities are observed due to large motional amplitudes caused by the coupling between the particle motion and cryostat vibrations. We have devised a cryogenic vibration isolation system based on an elastic pendulum that mitigates this effect and has enabled ring-down measurements of the center-of-mass motion that give quality factors up to 10^5. Furthermore, we have shown that the mechanical properties of the levitated particle can be controlled. We have tuned the trap frequencies from 30Hz to 180Hz by changing the current in the trap coils, and we have also demonstrated control over the motional amplitude of the particle motion via feedback using feedback coils in the chip to exert an additional magnetic force on the particle.
This thesis demonstrates magnetic levitation of superconducting microparticles on a chip as a novel platform for chip-based quantum experiments with um-sized particles and ultrasensitive force and acceleration sensors.
Martí Gutierrez Latorre
Chalmers, Microtechnology and Nanoscience (MC2), Quantum Technology
Chip-based superconducting traps for levitation of micrometer-sized particles in the Meissner state
Superconductor Science and Technology,; Vol. 33(2020)
A chip-based superconducting magnetic trap for levitating superconducting microparticles
IEEE Transactions on Applied Superconductivity,; Vol. 32(2022)
In this thesis, we have laid the groundwork required to perform such experiments. We started by building a model to accurately simulate levitated superconductors in magnetic traps of complex geometries. We used this model to design magnetic traps based on coils made of superconducting material, fabricated on silicon chips. Then, we developed microfabrication processes to manufacture such chip traps and microparticles of superconducting materials such as niobium and lead in the shape of disks, rings, and spheres of tens of micrometers in size.
We used the magnetic chip traps to levitate these spheres, in vacuum, at temperatures near absolute zero using a dilution refrigerator. To study the particle motion, we tracked the levitated spheres with a camera using a cryogenic-compatible custom-made microscope, which demonstrated the levitation of a microparticle on a chip for a few seconds when under illumination. Using a magnetic field sensor instead of light to measure the particle motion allowed the particles to levitate over days. Thanks to this long stability, we could thoroughly characterize the system and show that our model properly describes the particle motion in the magnetic trap and its detection. We observed that the amplitude of the particle motion is increased by mechanical vibrations, caused by the dilution refrigerator. To decrease the amplitude of motion, we devised a vibration isolation system that isolates the levitated particle from the mechanical vibrations in the refrigerator by hanging the experiment from a pendulum inside the refrigerator. To further reduce the amplitude of motion, we have implemented a means of particle motion control, based on the application of a magnetic feedback force on the particle. We have shown the proof-of-principle of this approach and studied the possibility of using it to bring the particle motion into the quantum regime.
The results in this thesis provide a stepping stone towards quantum experiments, as well as force and acceleration sensing using magnetically levitated superconducting microparticles.
Continuously Monitored Quantum Sensors: Smart Tools and Applications
Swedish Research Council (VR) (2019-00390), 2020-01-01 -- 2022-12-31.
Levitated superconducting mechanical resonators: a novel platform for quantum experiments and sensing
Knut and Alice Wallenberg Foundation (2019.0231), 2020-07-01 -- 2025-06-30.
Exploring nonclassical states of center-of-mass mechanical motion with superconducting magneto- and levitomechanics
European Commission (EC) (EC/HE/101080143), 2022-10-01 -- 2026-09-30.
Areas of Advance
Nanoscience and Nanotechnology
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5253
Kollektorn, lecture room, MC2-huset, Campus Johanneberg
Opponent: Prof. Dr. Tjerk Oosterkamp, Leiden Instituut Onderzoek Natuurkunde, Netherlands