Interactions of Intrinsic Quantum Defects with a Nanoelectromechanical System

Nanoelectromechanical systems (NEMS) resonators operating in the quantum regime provide a powerful platform for investigating mechanical motion at its most fundamental level. Their intrinsic ability to couple to environmental degrees of freedom, along with the long coherence times of their mechanical excitations (phonons), makes them particularly promising for quantum information and sensing applications. However, in this regime, the performance of NEMS resonators are predominantly affected by intrinsic material defects, acting as two-level systems (TLS). These quantum defects, ubiquitous in solid-state quantum devices at low temperatures, can exchange energy with their host field, causing dissipation and noise. Despite these dominant effects, the microscopic origin of such quantum defects is still unknown, and their interactions with phononic devices have been elusive. Here, we present a detailed investigation into these interactions between quantum defects and phonons within piezoelectric lithium niobate NEMS resonator shielded by phononic crystals. We identify TLS defects as the primary source of excess noise at millikelvin temperatures, limiting their performance and sensitivity. By controlling the TLS frequency in situ with the application of electric field---and strain field due to piezoelectricity---we demonstrate strong resonant coupling between a mechanical mode of our NEMS resonator and a single, intrinsic TLS. Varying the resonant drive and/or temperature allows controlled ascent of the nonequidistant energy ladder and reveals the dressed states of the hybridized system. Fluctuations of the TLS on and off resonance with the mode induces switching between dressed and bare states; this elucidates the complex quantum nature of TLS-like defects in mesoscopic systems. We demonstrate that individual TLS defects can be precisely controlled and manipulated, transforming them from detrimental dissipation and noise sources into valuable quantum resources. The ability to harness this intrinsic nonlinearity of a nanomechanical resonator with quantum defects offers new directions towards quantum sensing and information.