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Journal of Physics D Applied Physics

Losses in magnetic field resilient coplanar stripline resonators

Ivo P. C. Cools, Rodrigo Lopez Baez, Vittorio Buccheri, O. O. Shvetsov, Nermin Trnjanin, Emil Hogedal, Saroj P. Dash

Published May 30, 2025• 3 citations
Publisher

Abstract

Abstract Coplanar waveguide (CPW) resonators have shown considerable success in circuit quantum electrodynamics research. However, their susceptibility to an external magnetic field limits their general functionality. In addition, electrical voltage gating for devices coupled to a CPW resonator necessitates alterations to the ground plane, changing the resonator characteristics. To avoid these problems, we fabricated and measured quarter wavelength differentially driven coplanar stripline (CPS) resonators made of NbTiN. We measured multiple devices with varying geometrical parameters in these novel resonators and analyse changes in their internal quality factor and two-level system losses. Furthermore, we establish the resilience of the resonators to an applied magnetic field by tracking the resonance frequency shift, without optimisation of the resonator geometry. Due to their flexible design and high magnetic field resilience, CPS resonators are particularly well-suited to study superconductor-semiconductor heterophysics where voltage gating or a magnetic field are necessary.

DOI
10.1088/1361-6463/added5

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Abstract Coplanar waveguide (CPW) resonators have shown considerable success in circuit quantum electrodynamics research. However, their susceptibility to an external magnetic field limits their general functionality. In addition, electrical voltage gating for devices coupled to a CPW resonator necessitates alterations to the ground plane, changing the resonator characteristics. To avoid these problems, we fabricated and measured quarter wavelength differentially driven coplanar stripline (CPS) resonators made of NbTiN. We measured multiple devices with varying geometrical parameters in these novel resonators and analyse changes in their internal quality factor and two-level system losses. Furthermore, we establish the resilience of the resonators to an applied magnetic field by tracking the resonance frequency shift, without optimisation of the resonator geometry. Due to their flexible design and high magnetic field resilience, CPS resonators are particularly well-suited to study superconductor-semiconductor heterophysics where voltage gating or a magnetic field are necessary.
Superconducting coplanar-waveguide (CPW) resonators are one of the key devices in circuit quantum electrodynamics (cQED). Their performance can be limited by dielectric losses in the substrate and in the material interfaces. Reliable modeling is required to aid in the design of low-loss CPW structures for cQED. We analyze the geometric dependence of the dielectric losses in CPW structures using finite-element modeling of the participation ratios of the lossy regions. In a practical scenario, uncertainties in the the dielectric constants and loss tangents of these regions introduce uncertainties in the theoretically predicted participation ratios. We present a method for combining loss simulations with measurements of two-level-system-limited quality factors and resonance frequencies of CPW resonators. Namely, we solve an inverse problem to find model parameters producing the measured values. High quality factors are obtainable by properly designing the cross-sectional geometries of the CPW structures, but more accurate modeling and design methods for low-loss CPW resonators are called for major future improvements. Our nonlinear optimization methodology for solving the aforementioned inverse problem is a step in this direction.
We successfully designed and 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 is short 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. The obtained loaded quality factors are significantly high, i.e. up to ∼10 6 . When the temperature increases slowly from the base temperature (20 mK), the resonance frequencies of the resonators are blue shifted and the quality factors are lowered slightly. In principle, this type of device can integrate a series of CPW resonators with a common feedline, making it a promising candidate as the data bus for coupling distant solid-state qubits and the sensitive detector of single photons.
Superconducting coplanar waveguide resonators that can operate in strong magnetic fields are important for a variety of high-frequency superconducting devices. Magnetic fields degrade resonator performance by creating Abrikosov vortices that cause resistive losses and frequency fluctuations, or suppress the superconductivity entirely. To mitigate these effects, the authors investigate how device geometry and lithographically defined artificial defects can control vortex dynamics. These techniques allow the resonators to retain single-photon quality factors of about 105{}^{5}5 at ${B}_{\ensuremath{\parallel}}$ \ensuremath{\simeq} 6 T, for fast charge readout of a gate-defined double quantum dot at ${B}_{\ensuremath{\parallel}}$ = 1 T.
We characterize niobium nitride (NbN) λ/2 coplanar waveguide resonators, which were fabricated from a 10-nm-thick film on silicon dioxide grown by sputter deposition. For films grown at 180 °C, we report a superconducting critical temperature of 7.4 K associated with a normal square resistance of 1 kΩ, leading to a kinetic inductance of 192 pH/◻. We fabricated resonators with a characteristic impedance up to 4.1 kΩ and internal quality factors Qi>104 in the single photon regime at zero magnetic field. Moreover, in the many photon regime, the resonators present a high magnetic field resilience with Qi>104 in a 6 T in-plane magnetic field and in a 300 mT out-of-plane magnetic field. These findings make such resonators a compelling choice for circuit quantum electrodynamics experiments involving quantum systems with small electric dipole moments operated in finite magnetic fields.
High-quality superconducting coplanar waveguide (SCPW) resonators are crucial for developing superconducting quantum information devices and sensors. We designed quarter-wavelength SCPW resonators and fabricated the SCPW resonators using Nb thin film. The resonant characteristics were measured at T = 4.2 K, revealing the intrinsic quality factor and the coupling quality factor to be Qi = 4,784 and Qc = 17, 980, respectively. Our design and fabrication techniques would be very useful to develop a gate-tunable superconducting qubit based on the semiconductor nanostructures.
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.