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arXiv:2503.07431 · Applied Physics Letters

Fabrication and characterization of vacuum-gap microstrip resonators

Christian Schlager, Romain Albert, Gerhard Kirchmair

Published Aug 18, 2025• 0 citations
arXivHTMLPDF

Abstract

In traveling-wave parametric amplifiers (TWPAs), low-loss capacitors are necessary to provide 50 Ω impedance matching to the increased inductance that is brought in by the nonlinear elements used for amplification, be it Josephson junctions or high kinetic inductance materials. Here, we report on the development of a fabrication process for vacuum-gap microstrips, a design in which the ground plane is suspended in close proximity above the center conductor without the support of a dielectric. In addition to high-capacitance transmission lines, this architecture also enables air-bridges and compact parallel-plate capacitors. The performance of the fabrication is examined using distributed aluminum and granular aluminum resonators in a cryogenic dilution refrigerator setup, showing quality factors on par with the fabrication processes used in state-of-the-art TWPAs. In addition to characterizing the dependence of the quality factors on power, also their behavior with respect to temperature is explored, applying a model based on thermal quasi-particles and saturable two-level systems (TLSs), showing that the quality factors of the resonators are limited by TLS.

DOI
10.1063/5.0269702

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In traveling-wave parametric amplifiers (TWPAs), low-loss capacitors are necessary to provide 50 Ω impedance matching to the increased inductance that is brought in by the nonlinear elements used for amplification, be it Josephson junctions or high kinetic inductance materials. Here, we report on the development of a fabrication process for vacuum-gap microstrips, a design in which the ground plane is suspended in close proximity above the center conductor without the support of a dielectric. In addition to high-capacitance transmission lines, this architecture also enables air-bridges and compact parallel-plate capacitors. The performance of the fabrication is examined using distributed aluminum and granular aluminum resonators in a cryogenic dilution refrigerator setup, showing quality factors on par with the fabrication processes used in state-of-the-art TWPAs. In addition to characterizing the dependence of the quality factors on power, also their behavior with respect to temperature is explored, applying a model based on thermal quasi-particles and saturable two-level systems (TLSs), showing that the quality factors of the resonators are limited by TLS.
Abstract It is expected that the operation of microwave kinetic inductance travelling wave parametric amplifiers (KITWPAs) can be extended to the millimetre (mm) and the sub-mm wavelength range as long as the frequency is below the gap frequency of the superconducting film. This paper presents possible mm-wave designs for KITWPAs based on microstrip transmission lines. Our device is designed based on the BCS (Bardeen–Cooper–Schrieffer) model which successfully reproduces the measured transmission profile, gain, bandwidth, and nonlinear response of a fabricated KITWPA operating in the Ka-band, and includes the millimetre-wave dielectric loss determined by fitting the quality factor of a W-band microstrip resonator. We suggest a layout for a KITWPA operating near 220 GHz that can be fabricated using the same superconducting properties as the Ka-band device and can be coupled to a waveguide system. We conclude the paper by extending the 220 GHz design to higher frequency regimes approaching 1 THz.
Josephson parametric amplifiers have become a critical tool in superconducting device physics due to their high gain and quantum-limited noise. Traveling wave parametric amplifiers (TWPAs) promise similar noise performance, while allowing for significant increases in both bandwidth and dynamic range. We present a TWPA device based on an LC-ladder transmission line of Josephson junctions and parallel plate capacitors using low-loss amorphous silicon dielectric. Crucially, we have inserted λ/4 resonators at regular intervals along the transmission line in order to maintain the phase matching condition between pump, signal, and idler and increase gain. We achieve an average gain of 12 dB across a 4 GHz span, along with an average saturation power of −92 dBm with noise approaching the quantum limit.
In the field of circuit quantum electrodynamics (c-QED), the coherent interaction of two-level systems (TLSs) with photons, confined in a superconducting microwave\nresonator, opens up new possibilities for quantum computing experiments.\nThis thesis contributes to the expansion of c-QED tool-box with slow propagation line-based solutions for ubiquitous techniques, such as cryogenic Near Field Scanning Microwave Microscopy (NSMM), Electron Spin Resonance (ESR) spectroscopy and Traveling Wave Parametric Amplifier (TWPA). For NSMM, novel compact superconducting fractal resonators have been developed to be directly integrated on a scanning probe. We report NSMM operation based on a microwave high-Q resonator populated with less than 10^3 photons and demonstrate a capacitive sensitivity of 0.38 aF/rtHz.\nThe unique properties and the design flexibility of fractal resonators also boost their resiliency to strong magnetic fields for ESR studies. The reported high Q-factors above 10^5 in a magnetic field up to 0.4 T translate into ESR sensitivity of 5 10^5 spins/rtHz.\nFurthermore, we demonstrate the operation of a practical TWPA based on a slow propagation fractal line. We achieve per unit length gain of > 0.5 dB/cm and total gain of ~6 dB for a 10 cm long line. Due to a radically shortened line, the amplifier is less vulnerable to fabrication defects. Moreover, due to a successful impedance matching\nbetween the amplifier line and in/out terminals, the obtained gain vs frequency characteristic has only moderate ripples. To mitigate a common TWPA problem of\ncoupling to parasitic ground plane resonances, we deploy a novel multilayer fabrication technology, which combines high and low kinetic inductance (KI) elements.\nFinally, we present an alternative implementation of a slow propagation line: a microstrip line with a thin film Atomic Layer Deposition (ALD) Al2O3 oxide. The resonator, based on a segment of a microstrip line, has a Q-factor on the order of 10^4 at single photon powers, reaching up to 10^5 at higher powers. As an additional functionality,\nwe incorporate dc current control over KI so that the resonance frequency is tuned by 62 MHz range, which corresponds to a KI-related nonlinearity of 3%.
We present a detailed simulation and design framework for realizing traveling wave parametric amplifiers (TWPAs) using the nonlinear kinetic inductance of disordered superconductors -- in our case niobium-titanium-nitride (NbTiN). These kinetic inductance TWPAs (KITs) operate via three-wave mixing (3WM) to achieve high broadband gain and near-quantum-limited (nQL) noise. Representative fabricated devices -- realized using an inverted microstrip (IMS), dispersion-engineered, artificial transmission line -- demonstrate power gains above 25 dB, bandwidths beyond 3 GHz, and achieve ultimate system noise levels of 1.1 quanta even when operated with no magnetic shielding. These performance metrics are competitive with state-of-the-art Josephson-junction-based TWPAs but involve simpler fabrication and able to providing three orders of magnitude higher dynamic range (IIP1=−68IIP_1 = -68IIP1​=−68 dBm, IIP3=−55IIP_3 = -55IIP3​=−55 dBm), and high magnetic field resilience -- making KITs an attractive technology for highly multiplexed readout of quantum information and superconducting detector systems.
The performance and scalability of superconducting quantum circuits depends critically on the microwave environment. Minimizing signal reflections and suppressing thermal noise are essential for achieving high-fidelity readout and preserving qubit coherence. A significant challenge arises from the use of conventional cryogenic components such as isolators and circulators, which exhibit nonideal out-of-band reflection characteristics. Reflections degrade impedance matching and limit the performance of broadband quantum limited amplifiers. Superconducting implementations of reflectionless microwave filters offer a promising solution to mitigate these issues. The focus of this work is the fabrication and cryogenic characterization of reflectionless filters compatible with superconducting qubit fabrication flows. Devices were implemented on high resistivity silicon substrates using aluminum ground planes, integrated nichrome resistors, and crossovers formed with SiO2 interlayer dielectric. Cryogenic measurements at 20 mK demonstrate high return loss, confirming the viability of these filters for co-fabrication with traveling-wave parametric amplifiers (TWPAs) and circuit quantum electrodynamics (cQED) architectures. The filters exhibit low insertion loss in the passband to maintain quantum measurement efficiency and provide broadband reflection suppression across frequencies relevant to superconducting qubits, offering a scalable way to manage microwave noise in superconducting quantum processors.
Ultralow-noise microwave amplification and detection play a central role in different applications, going from fundamental physics experiments to the deployment of quantum technologies. In many applications the necessity of reading multiple detectors, or cavities or qubits, calls for large bandwidth amplifiers with the lowest possible noise. Current technologies are based on High Electron Mobility Transistors and Josephson Parametric Amplifiers. Both have limitations, the former in terms of the minimum noise, the latter in terms of bandwidth. Superconducting Traveling Wave Parametric Amplifiers (TWPAs) have the potential of offering quantum limited noise and large bandwidth. These amplifiers are based on the parametric amplification of microwaves traveling along a transmission line with embedded nonlinear elements. We are developing superconducting TWPAs based both on Josephson junction arrays (Traveling Wave Josephson Parametric Amplifiers) and on nonlinear kinetic inductance (Dispersion Engineered Traveling Wave Kinetic Inductance Amplifiers). Our goal is to achieve large bandwidth (in the 5 to 10 GHz range), large gain (more than 20 dB), large saturation power (more than −50 dBm), and near quantum limited noise (noise temperature less than 600 mK). Current achievements in the design and development of the high performance TWPAs are here reported and discussed, together with current limitations and possible future developments.