High-Q quartz phononic crystal resonators for quantum memory

Quantum memory is a crucial building block for the quantum internet. Piezoelectric mechanical resonators are good candidates for scalable quantum memory units due to their long lifetime and compact size. We fabricated suspended 1-D Z-cut quartz phononic crystal resonators with a width-extension defect mode at around 100 MHz. We measured a Q of up to 6.8 x10⁵ at 8 K, with a corresponding lifetime of about 1.0 ms. Such modes can be piezoelectrically coupled to superconducting quantum systems like SNAIL (Superconducting Nonlinear Asymmetric Inductive eLement) and fluxonium. The coupling electrodes will be implemented on a chip housing the superconducting circuits, which will be flip-bonded to the mechanical resonator chip with a micron-scale air gap. Such non-contact bonding eliminates mechanical losses in the metal electrodes and two-level system losses at the electrode-resonator interface. This modular integration architecture will improve the flexibility and scalability of the whole hybrid system. We present simulations estimating coupling rate of the mechanical modes to other systems and discuss strategies to improve it.