Our best balances are limited by the thermal noise (Brownian motion) in the torsion fiber. To achieve improved performance, we have built a new balance which is cooled to ~5 K by a pulse tube refrigerator and housed in multiple layers of magnetic and thermal shielding, including a layer of superconducting lead. This torsion balance exhibited torque noise levels lower than room temperature thermal noise by a factor of about four. 

We searched for violations of the weak equivalence principle using this cryogenic torsion balance with a pendulum comprised of superconducting niobium and copper. This test set upper limits on EP violations for Cooper pairs of 9.2 x 10^-4.

Relevent Publications:

"Test of the Equivalence Principle for Superconductors,” M.P. Ross, S.M. Fleischer, I.A. Paulson, P. Lamb, B.M. Iritani, E.G. Adelberger, C.A. Hagedorn, K. Venkateswara, C. Gettings, E.A. Shaw, S.K.Apple, J.H. Gundlach (2024)

"A Cryogenic Torsion Balance Using a Liquid-Cryogen Free, Ultra-Low Vibration Cryostat," S. M. Fleischer, M. P. Ross, K. Venkateswara, C. A. Hagedorn, E. A. Shaw, E. Swanson, B. R.Heckel, J. H. Gundlach. Review of Scientific Instruments (2022)