Geometric energy transfer in a St\"uckelberg interferometer of two parametrically coupled mechanical modes.

Geometric phase, which is acquired after a system undergoing cyclic evolution in the Hilbert space, is believed to be noise-resilient because it depends only on the global properties of the evolution path. Here, we report geometric control of energy transfer between two parametrically coupled mechanical modes in an optomechanical system. The parametric pump is controlled along a closed loop to implement geometric St\"uckelberg interferometry of mechanical motion, in which the dynamical phase is eliminated using the Hahn-echo technique. We demonstrate that the interference based solely on the geometric phase is robust against certain noises. And, more remarkably, we show that the all-geometric approach can achieve a high energy-transfer rate that is comparable to those using conventional dynamical protocols.