Strong coupling of a transmon qubit and a phononic crystal cavity array*

Coupling superconducting circuits to nanomechanical systems opens up new opportunities in quantum acoustics (e.g. QND phonon detection) and the exploration of technologically-relevant devices such as quantum memory elements. In particular, phononic crystal cavities (PCCs), which localize sound at the wavelength scale, show great promise as ultra-compact, long-lived mechanical elements which can be strongly coupled to a superconducting circuit despite being orders of magnitude smaller. Building upon our previous work, we now demonstrate coupling of a transmon qubit to an array of PCCs with mode frequencies in the 2.0-2.4 GHz range. We fabricate the qubit and associated control and readout circuitry on a silicon substrate and the PCCs on a suspended thin film of lithium niobate, effectively forming an array of one-dimensional “phononic wires” with localized resonances at a set of precisely engineered frequencies. The phononic defects sites are coupled to the qubit via superconducting electrodes patterned directly on top of the cavity mirrors. We measure qubit-phonon coupling rates in excess of 10 MHz, putting the system well in the strong coupling regime.