Optically heralded microwave photons - Part 2

Integrated piezo-optomechanical systems are a promising platform for microwave-to-optical quantum frequency conversion. Because of the inherent impedance mismatch between the nanoscale GHz mechanical mode and microwave transmission line, an intermediary microwave resonance is utilized to enhance the external coupling rate. The microwave resonances are formed by the standing waves in a high-impedance (high-Z) superconducting waveguide on a niobium-titanium-nitride (NbTiN) platform. To simplify fabrication and minimize optically induced quasiparticle generation, the piezo-optomechanical element and the microwave circuit are fabricated on separate chips and heterogeneously integrated via cross-chip wirebonds. The large kinetic inductance from NbTiN nanowires enables magnetic frequency tunability allowing us to bring one microwave mode in resonance with the mechanical mode, enhancing the conversion efficiency. Beyond the quantum frequency conversion application, such a high-Z waveguide provides a convenient and flexible way of enhancing the external coupling rate of various piezo-electric nanomechanical systems.