Qubit-assisted transduction for a detection of surface acoustic waves near the quantum limit*

Surface acoustic waves (SAW) have recently attracted much interest for hybrid quantum systems as an alternative quantum mode localized on a surface of a material. In piezoelectric materials, SAW can be strongly coupled to electric fields and are widely applied in compact microwave components because of their short wavelength and small losses. SAW can also couple to other physical systems such as superconducting qubit , quantum dots, NV centers, and optical systems through various form of elastic effects.
Here, we report hybrid quantum system consisting of a SAW resonator, a superconducting qubit, and a MW resonator. We demonstrate microwave-driven parametric couplings induced by the nonlinearity of the qubit, which serves as an interface between the phonons in the SAW resonator and the photons in the MW resonator. The thermal phonons in the sub-GHz SAW resonator are up-converted to the MW frequency where near-quantum-limited measurement of photons is available. We observe thermal fluctuations in the SAW resonator below the mean phonon number of unity with an unprecedented sensitivity.