Electrostatic coupling of microwave photons and phonons in a silicon phononic crystal resonator

Mechanical oscillators with long energy relaxation lifetimes possess significant potential for quantum information processing, in the form of quantum memories and transducers. We present the design and measurement of an electromechanical system, which generates interactions between resonant GHz frequency microwave photons and acoustic phonons on a silicon-on-insulator platform. The system is based on electrostatic transduction, where the application of an external electric field on a moving capacitor engenders an electromechanical interaction. Bandgap engineering of the phononic crystal resonator combined with the avoidance of lossy piezoelectric materials gives rise to significant phonon lifetimes, reaching 265μs (Q ∼ 10 million at 5GHz). A tunable high impedance TiN microwave resonator ensures the resonance condition while simultaneously boosting the electromechanical interaction, which can be parametrically enhanced to reach g/2π = 1.1MHz, sufficient for the system to enter the strong coupling regime. Mode thermometry measurements conducted at mK temperatures indicate the absence of any significant drive induced heating, despite the large electric fields applied to the device, with both mechanical and microwave resonators remaining in their ground state. The combination of long lifetime, large coupling strength and ground state operation establishes our system in a favorable position for future quantum acoustics and transduction experiments.