High-Q bulk acoustic resonator for building long-lived Quantum Memory

Hybrid system coupling superconducting qubits and high-Q mechanical resonators can enhance coherence times and scalability. With a long phonon lifetime, the resonators can serve as information storage units. Here, we attempt to fabricate a quartz phononic cavity resonator with a high Q and small mode volume. Our resonator is a cuboid bulk acoustic wave (BAW) resonator suspended by 1-D chain phononic crystal tethers. The chain is designed so that the target mechanical mode lies within the band gap of the phononic crystal, localizing the mode in the central defect and effectively eliminating acoustic radiation losses. Our quartz-only chip will be flip-chip bonded to another chip which contains electrodes for piezoelectrically coupling phonons to a superconducting quantum microwave circuit. Our mechanical devices are designed with resonance frequencies around 100 MHz, much smaller than microwave qubit frequencies. These 10s of micron devices are much more robust to fabrication and intrinsic defects compared to submicron microwave phononic devices. We also believe that gearing down the frequency of our information storage will sustain longer lifetimes. The frequency mismatch between phonons and photons will eventually be addressed with a nonlinear microwave parametric converter.