Fig. 2

The gravity sensor of the superconducting gravimeter. a Superconducting sphere and coils. b Vertical force applied to the levitating sphere as a function of vertical position. The vertical force gradient is made weak for a limited interval of the vertical coordinate (z). Ideally, the force should be linearly dependent on z. In reality, there exists a small but finite quadratic term, which gives rise to nonlinear responses of the gravimeter