Surface Modification and Coherence in Lithium Niobate SAW Resonators

TL;DR

This paper investigates how surface treatments affect the coherence of lithium niobate SAW resonators at cryogenic temperatures, revealing complex relationships between surface quality and decoherence mechanisms.

Contribution

It provides a detailed experimental analysis of surface modification effects on resonator coherence, highlighting unexpected correlations between surface quality and TLS density.

Findings

Surface treatments can alter TLS density in lithium niobate resonators.

Improved surface quality does not always correlate with reduced TLS density.

Surface modification techniques influence decoherence mechanisms in quantum acoustic devices.

Abstract

Lithium niobate is a promising material for developing quantum acoustic technologies due to its strong piezoelectric effect and availability in the form of crystalline thin films of high quality. However, at radio frequencies and cryogenic temperatures, these resonators are limited by the presence of decoherence and dephasing due to two-level systems. To mitigate these losses and increase device performance, a more detailed picture of the microscopic nature of these loss channels is needed. In this study, we fabricate several lithium niobate acoustic wave resonators and apply different processing steps that modify their surfaces. These treatments include argon ion sputtering, annealing, and acid cleans. We characterize the effects of these treatments using three surface-sensitive measurements: cryogenic microwave spectroscopy measuring density and coupling of TLS to mechanics, x-ray photoelectron spectroscopy and atomic force microscopy. We learn from these studies that, surprisingly, increases of TLS density may accompany apparent improvements in the surface quality as probed by the latter two approaches. Our work outlines the importance that surfaces and fabrication techniques play in altering acoustic resonator coherence, and suggests gaps in our understanding as well as approaches to address them.