Towards coupled HBARs for multimode quantum acoustics.

Superconducting circuits provide a powerful platform for controlling and detecting mechanical motion at the quantum level. Among mechanical systems, high-overtone bulk acoustic resonators (HBARs) have recently emerged as promising candidates due to their long coherence times and high degree of controllability. Scaling these systems toward multiple coupled HBARs could enable the engineering of artificial phononic band structures coupled to superconducting qubits. However, achieving strong inter-resonator coupling remains challenging due to the limited piezoelectric coupling and small energy participation ratio in the piezoelectric layer.

Here, we present a novel HBAR geometry that enables coupling strengths of several megahertz between neighboring resonators. We also report on multi-HBAR devices fabricated on thin substrates, exhibiting frequency mismatch below 0.015%. Realizing resonant and spatially separated HBARs represents a crucial step toward constructing chains of strongly coupled mechanical resonators for quantum acoustic applications.