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Superconductor Science and Technology

Fabrication and characterization of electrically tunable high-Tcsuperconducting resonators incorporating barium strontium titanate as a tuning material

E-H Lee, Junghyun Sok, S-J Park, J-S Lee, IH Song, June Sik Kwak, K-R Jung, Jy Kim, Sy Yoon, Duk Young Jeon

Published Nov 1, 1999• 4 citations
Publisher

Abstract

Fabrication and characterization of electrically tunable high-Tc superconducting resonators incorporating barium strontium titanate as a tuning material, E-H Lee, J Sok, S-J Park, J-S Lee, I-H Song, J Kwak, K-R Jung, J-Y Kim, S-Y Yoon, D Y Jeon
DOI
10.1088/0953-2048/12/11/385

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Radio-frequency reflectometry allows for fast and sensitive electrical readout of charge and spin qubits hosted in quantum dot devices coupled to resonant circuits. Optimizing readout, however, requires frequency tuning of the resonators and impedance matching. This is difficult to achieve using conventional semiconductor or ferroelectric-based varactors in the detection circuit as their performance degrades significantly in the mK temperature range relevant for solid-state quantum devices. Here we explore a different type of material, strontium titanate, a quantum paraelectric with exceptionally large field-tunable permittivity at low temperatures. Using strontium titanate varactors we demonstrate perfect impedance matching and resonator frequency tuning at 6 mK and characterize the varactors at this temperature in terms of their capacitance tunability, dissipative losses and magnetic field sensitivity. We show that this allows us to optimize the radio-frequency readout signal-to-noise ratio of carbon nanotube quantum dot devices to achieve a charge sensitivity of 4.8 μμμe/Hz1/2^{1/2}1/2 and capacitance sensitivity of 0.04 aF/Hz1/2^{1/2}1/2.
Here, we demonstrate an on-chip cryogenic microwave frequency-comb on a planar superconducting resonator fabricated on Strontium titanate(STO), a quantum paraelectric material. The material's Kerr-type nonlinearity arising from the quantum paraelectric phase, an underexplored state, enables pronounced resonance shifts, Duffing-like bifurcations and comb generation via cascaded foure-wave mixing. Our results establish STO resonators as a low power, cryogenically compatible platforms for nonlinear microwave photonics, with applications in scalable quantum control and on-chip frequency synthesis.
Ferroelectric materials, with their strong nonlinearities, underpin key technologies across radio-frequency (RF) signal processing, optical communications, and emerging quantum systems. Barium titanate (BTO) is a notable example, combining strong piezoelectric and electro-optic responses. While bulk BTO has been studied for decades, the piezoelectric properties of its recently available thin films, and their behavior at the millikelvin temperatures relevant to quantum hardware, remain largely unexplored. Here, we fabricate and characterize surface acoustic wave (SAW) resonators on thin-film BTO. The measured devices exhibit high electromechanical coupling (k2eff 0.14 at 5.2 GHz) and operate up to 7.8 GHz. From these measurements, combined with finite-element modeling of the multi-domain microstructure, we extract an effective piezoelectric coefficient d33eff of 53 pC/N, comparable to bulk BTO. Exploiting the intrinsic ferroelectricity, we further demonstrate low-voltage switching with a fast (100 ns) response, attractive for reconfigurable RF front-ends and parametric amplifiers. Extending these measurements to millikelvin temperatures, we find that the piezoelectric response persists, with d33eff 19 pC/N, pointing to the potential of BTO for piezoelectric coupling in superconducting quantum circuits. These results position thin-film BTO as a promising piezoelectric platform for both classical and quantum information technologies.