Loss channels affecting lithium niobate phononic crystal resonators at cryogenic temperature

TL;DR

This study explores how loss mechanisms affect lithium niobate phononic crystal resonators at cryogenic temperatures, revealing the impact of material and design on their quality factors and potential for quantum circuit integration.

Contribution

It provides new insights into loss channels in lithium niobate phononic resonators at millikelvin temperatures, highlighting the influence of material choice and defect engineering.

Findings

Achieved high internal quality factors ($Q_i$ above 10^5 - 10^6) at millikelvin temperatures.

Observed signatures of the phononic bandgap through quality factor measurements.

Identified the impact of superconducting and TLS loss channels on device performance.

Abstract

We investigate the performance of microwave-frequency phononic crystal resonators fabricated on thin-film lithium niobate for integration with superconducting quantum circuits. For different design geometries at millikelvin temperatures, we achieve mechanical internal quality factors $Q_i$ above $10^5 - 10^6$ at high microwave drive power, corresponding to $5\times10^6$ phonons inside the resonator. By sweeping the defect size of resonators with identical mirror cell designs, we are able to indirectly observe signatures of the complete phononic bandgap via the resonators' internal quality factors. Examination of quality factors' temperature dependence shows how superconducting and two-level system (TLS) loss channels impact device performance. Finally, we observe an anomalous low-temperature frequency shift consistent with resonant TLS decay and find that material choice can help to mitigate these losses.