Measurement of Microwave Loss in Complex Oxide Heterostructures for Hybrid Quantum Systems*

Hybrid microwave-acoustic systems at the quantum limit are emerging as a promising platform for the processing and storage of quantum information. In this area, piezo-acoustic cavities are of particular interest as they are capable of direct coupling of microwave to acoustic modes through piezoelectric modulation. However, creating a piezo-acoustic cavity requires on-chip integration of physically disparate piezoelectric and superconducting materials while maintaining a coherent behavior at microwave frequencies and milliKelvin (mK) temperatures. The extent of microwave loss in the piezoelectric elements within the cavities is specifically a point of concern. Here, we systematically study the microwave loss in epitaxial heterostructures of barium titanate (BTO)-on-silicon (BTO/Si) as a promising platform for piezo-acoustic cavities. We use a multi-resonator superconducting coplanar waveguide design as a pilot device to measure microwave losses at mK temperatures. By changing the thickness of various layers within the BTO/Si heterostructures including the buffer layers (e.g., YSZ, CeO2, STO, and LNO), we determine the contribution each oxide layer has to the overall loss of the heterostructure. Microwave transmission for each chip is measured at 30 mK-2 K with powers ranging from 0 to -60 dBm. The transmission spectra are then analyzed to extract the actual resonant frequency, quality factors (internal vs. external), and effective dielectric constant for each chip. The experimental work is complemented by COMSOL simulations that evaluate the electric field participation for each layer of the BTO/Si heterostructures.