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Chinese Physics B

Characterization of TLS losses of sputtered SiO x and AlO x films at subKelvin temperatures

Yu-Hao Hu, Zheng 争 Wang 王, Wei Geng, Hao Wu, Qiang Zhi, Jiang‐Qiao Ding, Jie Hu, Jing Li, Shengcai SHI

Published Feb 3, 2026• 0 citations
Publisher

Abstract

Abstract Superconducting low-loss transmission line resonators are crucial for various applications, including quantum computation and photon detection in submillimeter and far-infrared astronomy. Their performance is sensitive to both their geometry and material, with energy dissipation mainly due to two-level systems (TLS) located in amorphous dielectrics. This paper presents a lab-on-chip experiment to measure the TLS losses of sputtered SiO x and AlO x films in superconducting Nb coplanar waveguide (CPW) resonators at sub-Kelvin temperatures. Different CPW geometries are adopted for these experiments. The measurement results provide quantitative insights into TLS contributions in superconducting resonators, providing empirical evidence for material screening and circuit design.

DOI
10.1088/1674-1056/ae40e0

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Abstract Superconducting low-loss transmission line resonators are crucial for various applications, including quantum computation and photon detection in submillimeter and far-infrared astronomy. Their performance is sensitive to both their geometry and material, with energy dissipation mainly due to two-level systems (TLS) located in amorphous dielectrics. This paper presents a lab-on-chip experiment to measure the TLS losses of sputtered SiO x and AlO x films in superconducting Nb coplanar waveguide (CPW) resonators at sub-Kelvin temperatures. Different CPW geometries are adopted for these experiments. The measurement results provide quantitative insights into TLS contributions in superconducting resonators, providing empirical evidence for material screening and circuit design.
© 2019 American Physical Society. Superconducting quantum-computing architectures comprise resonators and qubits that experience energy loss due to two-level systems (TLSs) in bulk and interfacial dielectrics. An understanding of these losses is critical to improving performance in superconducting circuits. In this work, we present a method for quantifying the TLS losses of different bulk and interfacial dielectrics present in superconducting coplanar-waveguide (CPW) resonators. By combining statistical characterization of sets of specifically designed CPW resonators on isotropically etched silicon substrates with detailed electromagnetic modeling, we determine the separate loss contributions from individual material interfaces and bulk dielectrics. This technique for analyzing interfacial TLS losses can be used to guide targeted improvements to qubits, resonators, and their superconducting fabrication processes.
Superconducting coplanar waveguide (SCPW) resonators, key components for quantum computing and sensing applications, require a high internal quality factor ( Q i ) for effective qubit readout and quantum sensing applications. Minimizing two-level system (TLS) losses, particularly at material interfaces, is critical for gatemon and topological qubits operating at low temperatures and in high magnetic fields. NbTi, a superconducting alloy with a high upper critical field, enables SCPW resonators resilient to such conditions. We fabricated NbTi SCPW resonators on Si/SiO 2 substrates and systematically characterized their TLS-limited quality factors as functions of temperature and microwave photon number. Our results demonstrate that NbTi-based SCPWs on Si/SiO 2 substrates provide a promising platform for developing next-generation quantum circuits. • Successfully fabricated NbTi SCPW resonators on Si/SiO 2 substrates. • Characterized NbTi SCPW across temperature and photon ranges for quantum apps. • Achieved record TLS-limited quality factor Q TLS,0 = 9.21 × 10 4 for SCPW on Si/SiO 2 . • NbTi SCPW on Si/SiO 2 is promising for gatemon and topological qubits.
Superconducting quantum computing systems, including coplanar waveguide (CPW) resonators and qubits, are highly susceptible to energy dissipation from two-level systems (TLS) within bulk and interfacial dielectrics. CPW resonators serve as an ideal platform for characterizing these material losses at the single-photon excitation level. Building on recent experimental evidence that interface engineering can mitigate TLS losses, this study employs simulations to evaluate resonator quality factors across various interface modifications. Our results demonstrate that reducing losses at the substrate–air (SA) interface can increase the internal quality factor Qi by up to three orders of magnitude. While etching the SA interface also enhances Qi, material loss remains the dominant dissipation mechanism. Furthermore, we find that other lossy interfaces have a significantly smaller impact on the quality factor compared to the SA interface. These simulation results align with established experimental findings, providing a robust framework for refining resonator design. This work offers precise guidelines for TLS mitigation, essential for enhancing coherence times and developing more reliable superconducting quantum processors.
As superconducting kinetic inductance detectors (KIDs) continue to grow in popularity for sensitive submillimeter detection and other applications, there is a drive to advance toward lower-loss devices. We present measurements of diagnostic thin-film aluminum coplanar waveguide (CPW) resonators designed to inform ongoing KID development at NASA Goddard Space Flight Center. The resonance frequencies span f0f_0f0​ = 3.5-4 GHz and include quarter-wave and half-wave resonators with varying coupling capacitor designs. We present measurements of the device film properties and an analysis of the dominant mechanisms of loss in the resonators measured in a dark environment, demonstrating quality factors of Qi−1≈3.64−8.57 ×10−8Q_i^{-1} \approx 3.64-8.57\ \times 10^{-8}Qi−1​≈3.64−8.57 ×10−8. We observe an enhanced level of suppression in the loss contributions from two-level systems (TLS) at intermediate-to-high read powers, and a regime at these powers and low temperatures where contributions from intrinsic processes Qi−1Q_i^{-1}Qi−1​,other dominate the total loss. We also observe deviations from the standard TLS loss model at low powers and temperatures below 60 mK, and use a modified model to describe this behavior.
Tow-level system (TLS) loss in amorphous dielectric materials has been intensively studied at millikelvin temperatures due to its impact on superconducting qubit devices and incoherent detectors. However, the significance of TLS loss in superconducting transmission lines at liquid helium temperatures remains unclear. This study investigates TLS loss in amorphous SiO2 at liquid helium temperatures (about 4 K) within a frequency range of 130–170 GHz, using niobium microstrip and coplanar waveguide resonators. Our results demonstrate notable power and temperature dependence of dielectric loss, with the dielectric loss and quasiparticle loss exchanging dominance at around 4 K. These findings are consistent with TLS models and provide crucial insight for the design of superconducting devices operating at liquid helium temperatures.
Transmission losses in Nb superconducting coplanar waveguide (CPW) and microstrip lines were quantified at 2-mm wavelength and 3.3 K using a network analyzer. The loss measurement utilized both half-wavelength resonators and electrically long transmission lines. Validation of the measurement was achieved using a silicon membrane-based waveguide-to-CPW transition with low return loss across a broad bandwidth. Intrinsic losses were measured within a parametric space defined by frequency, temperature, signal power, and magnetic field. The multidimensional information allowed the breakdown of overall losses into major components, including quasiparticle loss, two-level system dielectric loss, vortex flow loss, and radiation loss. The performance of these transmission lines was extrapolated into the terahertz range based on these findings.