Superior frequency stability and long-lived state-swapping in cubic-SiC mechanical mode pairs

Abstract The multimode cavity optomechanical system offers versatile applications including state transduction, coherent interconnection, and many-body simulations. In this study, we developed a cavity electromechanical system that integrates a 3C-SiC membrane and a rectangular superconducting cavity to observe the generation of nearly degenerate pairs of mechanical modes. Subsequently, we derive the expression for intrinsic frequency under nonuniform stress and find that this method supports a remarkable resolution for stress analysis in thin films. Experimentally, we perform collective fitting on the measured set of 57 mechanical modes, revealing deviations in biaxial non-uniform stress on the order of MPa. These degeneracy-broken mechanical modes exhibit exceptional quality factors as high as $10^8$ in a thermal bath of 10 mK. Furthermore, Allan deviation indicates that these modes exhibit extremely stable frequencies compared with different types of optomechanical devices. We then performed state-swapping between near-degenerate mode pairs, demonstrating the transfer efficiency exceeding 78\%, attributed to their exceptionally long lifetimes. This study paves the way for the design of compact quantum phononic devices featuring high-quality-factor mechanical multimodes.