Towards a monolithic platform for coupling superconducting circuits to low-loss microwave phonons in AlScN on 4H-SiC

Hybrid quantum systems that combine superconducting qubits with acoustic resonators offer a promising path for scalable quantum information processing. While superconducting circuits provide strong nonlinearity and control, phonons offer compact footprints and long coherence times. This study introduces a monolithic platform using aluminum scandium nitride (AlScN) thin films on a silicon carbide (SiC) substrate, designed to integrate these two technologies without the complexity of flip-chip bonding or suspended structures.

The researchers developed a fabrication process that selectively removes AlScN from specific regions of the chip. This allows aluminum superconducting resonators to be patterned directly onto the exposed SiC, while the remaining AlScN regions serve as piezoelectric transducers for generating and routing microwave phonons. The team characterized the platform by measuring the internal quality factors of superconducting coplanar waveguide (CPW) resonators and the propagation loss of surface acoustic waves (SAW) at cryogenic temperatures.

The platform demonstrated successful integration of both components. The superconducting resonators achieved a coherent lifetime of 2.9 μs, showing that the SiC substrate and the selective etching process are compatible with superconducting quantum devices. Simultaneously, the AlScN-on-SiC regions exhibited low phononic propagation loss, corresponding to an estimated phonon lifetime of 7.6 μs. The study also identified that resonator performance is sensitive to surface treatment and substrate geometry, suggesting that bulk SiC dielectric loss and surface-related two-level systems are the primary bottlenecks for further improvement.

This work establishes AlScN-on-SiC as a viable, scalable alternative to existing hybrid quantum platforms. By avoiding the need for suspended phononics or complex bonding, this monolithic approach simplifies the design of large-scale quantum acoustic networks. The results provide a clear roadmap for future optimizations, such as improving SiC material purity and refining surface processing, to push this platform toward the coherence levels required for advanced quantum computing applications.