As an alternative to the diving board shaped force sensor, we are now developing a magnetically levitated small superconducting particle – or ‘Lead Zeppelin'' – as our mechanical resonator. We explore routes towards detection of gravity between small objects.

To measure ever smaller forces, we are turning to alternative experimental geometries. In our first attempts [1] we floated a superconducting lead sphere between two coils that were making an opposing magnetic field. In the middle between these coils the magnetic field is zero.

This is where the lead sphere would prefer to be due to the Meissner effect. This was tested at 4 K and demonstrated that SQUID detection is possible. However, to avoid the noise coming from the current source that powers the coils to generate the static magnetic field that levitates the particle, it is necessary to use a persistent current switch on the coils [2]. We found that at the frequencies of these resonators (<100 Hz) vibration isolation was the limiting factor.

Currently, we are investigating the ‘reversed’ geometry, in which a magnetic particle is sitting in a hole drilled in a superconductor. This technique looks very promising, and could provide unprecedented low damping rates, unheard of in earlier low frequency mechanical resonators [3].

References:
[1] PhD thesis Bob van Waarde [2016],  The Lead Zeppelin, a force sensor without a handle.
[2] van Waarde, Bob, Olaf Benningshof, and Tjerk Oosterkamp. "A magnetic persistent current switch at milliKelvin temperatures." Cryogenics 78 (2016): 74-77.
[3] Vinante, A., et al. "Ultralow mechanical damping with Meissner-levitated ferromagnetic microparticles." arXiv (2019): 1912.12252