Experimental demonstrations of high-Q superconducting coplanar waveguide resonators

Abstract: We designed and successfully fabricated an absorption-type of superconducting coplanar waveguide (CPW) resonators. The resonators are made from a Niobium film (about 160 nm thick) on a high-resistance Si substrate, and each resonator is fabricated as a meandered quarter-wavelength transmission line (one end shorts to the ground and another end is capacitively coupled to a through feedline). With a vector network analyzer we measured the transmissions of the applied microwave through the resonators at ultra-low temperature (e.g., at 20 mK), and found that their loaded quality factors are significantly high, i.e., up to 10^6. With the temperature increases slowly from the base temperature (i.e., 20 mK), we observed the resonance frequencies of the resonators are blue shifted and the quality factors are lowered slightly. In principle, this type of CPW-device can integrate a series of resonators with a common feedline, making it a promising candidate of either the data bus for coupling the distant solid-state qubits or the sensitive detector of single photons.

• The paper demonstrates a loaded quality factor of approximately 1.1654×10^6 for niobium superconducting CPW resonators measured at 20 mK.
• The methodology employs meandered quarter-wavelength designs with magnetron sputtering and photolithography to optimize impedance matching and resonance accuracy.
• The results highlight potential applications in quantum circuits and photon detection, showing effective frequency multiplexing and integration for sensor arrays.

Experimental Demonstrations of High-Q Superconducting Coplanar Waveguide Resonators

Introduction

The paper presents a meticulous experimental exploration into the fabrication and performance characteristics of high-quality superconducting coplanar waveguide (CPW) resonators made from niobium films deposited on silicon substrates. The main focus is the achievement of exceptionally high loaded quality factors, up to $Q_l \sim 10^6$, through precise engineering and low-temperature operations, leading to advanced potential applications in quantum information systems and photon detection mechanisms.

Design and Fabrication

The superconducting CPW resonators are crafted as meandered quarter-wavelength transmission lines, utilizing the established techniques of magnetron sputtering and photolithography. Niobium was chosen for its effective superconductivity at cryogenic temperatures, with a film thickness of approximately 160 nm, ensuring minimal loss. The overall CPW resonator length is set to achieve a fundamental resonance frequency of around 1.8575 GHz. The substrate's dielectric properties and resonator geometry are meticulously calibrated to optimize impedance matching and minimize reflective losses.

Measurement System

The resonators undergo rigorous testing using a vector network analyzer, leveraging ultra-low temperature conditions (20 mK). This setting allows for the precise characterization of transmission properties and resonance dip features. Optimal driving power conditions are established to maximize resonance dip depth and signal-to-noise ratio, crucial for analyzing quality factors.

Experimental Results

Results reflect a noteworthy loaded quality factor of $Q_l = 1.1654 \times 10^6$, calculated from the resonance frequency and 3dB bandwidth measurements. The resonance frequency demonstrates a temperature-dependent blue shift, accompanied by a slight reduction in the quality factor upon temperature increase, explained through the framework of two-level system (TLS) theories and substrate dielectric variations.

Additionally, the experiment supports the integration of multiple resonators on a single feedline, demonstrating effective frequency multiplexing capabilities. The precision in fabrication ensures minimal discrepancies between designed and measured resonance frequencies, confirming high manufacturing accuracy.

Discussion and Conclusions

The experimental achievements outlined underscore the potential of superconducting CPW resonators as integral components in large sensor arrays and quantum circuit implementations. Their high integrability and sensitivity to photon interactions suggest promising prospects in radiation detection and solid-state qubit coupling. Future advancements may focus on further enhancing the resonator integration density and expanding operational bandwidths, making these resonators adaptable to more complex quantum systems.

In summary, the exploration establishes a robust foundation for future superconducting CPW resonator applications, emphasizing their practicality in sensitive detection and quantum information processes. The paper invites further investigation into the optimization of these systems under varying operational conditions and in broader application contexts.