Novel torsion balance based on a spherical superconducting suspension

We report on the progress that has been made on the development of a spherical superconducting torsion balance in which the traditional fiber is replaced with a Meissner effect suspension operating at 4.2 K. The instrument can levitate a mass of 18 g in persistent mode and currently operates in a helium exchange gas pressure of a few mTorr. A superconducting rotation detector incorporating a superconducting quantum interference device magnetometer has been developed to interrogate the angular position of the torsion balance and provide its natural period. The natural period can be programmed from 25 s to 150 s by varying the current stored in the detector. The detector also offers the possibility of applying external torques onto the torsion balance for the purpose of servocontrol, and a simple derivative control has been developed. The properties of the rotation detector can be predicted from measurements of the inductances of the circuit components. The total measured torque noise is 2x10(-13) Nm/rootHz at around 0.02 Hz. This is two orders of magnitude higher than the intrinsic thermal noise and we are currently investigating possible sources of this excess noise. The highest quality factor that has been observed is 4x10(4) at a period of 40 s, and this corresponds to an ultimate sensitivity of 2x10(-16) Nm/rootHz. (C) 2004 American Institute of Physics.