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European Microwave Conference (EuMC)

Low-Temperature Superconducting MMIC Diplexers for RF Quantum AC-Voltage Standards

Abdulrahman H. A. Widaa, Oliver Kieler, Frauke Kathinka Helene Gellersen, Michael Haas, J. Kohlmann, Mark Bieler

Published Sep 23, 2025• 0 citations
Publisher

Abstract

With increasing applications of quantum systems operating at high frequencies, the role of integrated microwave components for quantum circuits becomes more and more important. In this article, we present low-temperature superconducting bandpass-bandstop diplexers for a new generation of quantum AC-voltage standards operating at 1 GHz. Based on stringent requirements, the diplexers are designed to provide low passband loss, good stopband rejection, and otherwise a broad ideally matched spectrum with excellent spurious-free performance. A second-order 1 GHz superconducting bandpass-bandstop diplexer is fabricated and characterized for verification. To this end, a cryogenic setup and on-chip calibration standards are implemented for S-parameter measurements. The experimental results exhibit low insertion loss as low as 0.03 dB in the passband section and high rejection in the bandstop branch exceeding 43 dB. Furthermore, the diplexer features wide spurious-free response extending to more than$14 \cdot \mathrm{f}_{0}$and occupies a very compact area of only$1.8 \text{mm} \times 1.2 \text{mm}$.

DOI
10.23919/eumc65286.2025.11235213

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With increasing applications of quantum systems operating at high frequencies, the role of integrated microwave components for quantum circuits becomes more and more important. In this article, we present low-temperature superconducting bandpass-bandstop diplexers for a new generation of quantum AC-voltage standards operating at 1 GHz. Based on stringent requirements, the diplexers are designed to provide low passband loss, good stopband rejection, and otherwise a broad ideally matched spectrum with excellent spurious-free performance. A second-order 1 GHz superconducting bandpass-bandstop diplexer is fabricated and characterized for verification. To this end, a cryogenic setup and on-chip calibration standards are implemented for S-parameter measurements. The experimental results exhibit low insertion loss as low as 0.03 dB in the passband section and high rejection in the bandstop branch exceeding 43 dB. Furthermore, the diplexer features wide spurious-free response extending to more than14⋅f014 \cdot \mathrm{f}_{0}14⋅f0​and occupies a very compact area of only1.8mm×1.2mm1.8 \text{mm} \times 1.2 \text{mm}1.8mm×1.2mm.
We demonstrate a decade-passband superconducting diplexer operating from dc to 27 GHz, integrated in a niobium, Josephson junction (JJ) compatible process. Both low-and high-pass branches of the diplexer are singly-terminated 5-pole Butterworth filters with a 2.5 GHz cutoff. Several diplexer test circuits were characterized with on-wafer cryogenic two-port calibration by terminating the third port of the device. Insertion loss below 0.8 dB are measured across the entire frequency range. Good agreement between measurements and simulations up to 27 GHz suggests that the upper diplexer band extends to the over-a-decade simulated 40 GHz. This device can be integrated in large-scale superconducting JJ-based quantum processors, broadband sensors, amplifiers and sources.
Protecting superconducting quantum circuits from nonideal return loss, including out-of-band circulator behavior, and enhancing the performance of broadband quantum-limited amplifiers can be accomplished using a superconducting version of a special class of microwave filters known as reflectionless filters. These filters can simultaneously permit low passband loss to preserve quantum efficiency and broadband reflectionless characteristics in the stop and pass bands. The filter also suppresses thermal photons emitted in its passband from the termination resistors by the nature of the dual-network topology. This work will review the application, theory, design, and modeling of a superconducting reflectionless filter, followed by fabrication details and the presentation of cryogenic performance measurements of a monolithic device. The filter was fabricated using Al on Si and incorporated NiCr resistors, allowing simple integration with other superconducting quantum devices. The filter with an area of 0.6 mm2achieves insertion loss of−0.75 dB-0.75{\,}\text {dB}−0.75dB(σ=0.24 dB\sigma = 0.24{\,}\text {dB}σ=0.24dB), including its connectorized package over a 80% fractional bandwidth centered at 8 GHz, and 10-dB packaged return loss from dc to above 14.5 GHz.
An ultra-wide stopband hairpin bandpass filter with integrated nonuniform transmission lines was designed and fabricated for highly sensitive measurements at cryogenic temperatures down to millikelvin and a frequency range of 10 Hz-10 GHz. The scattering matrices of the filter were characterized at T = 4.2 K. The filter provides a stopband from 10 Hz to 2.2 GHz and from 2.3 GHz to 10 GHz with more than 50 dB and 40 dB of amplitude suppression, respectively. The center frequency of the passband is f0 = 2.25 GHz with a bandwidth Δf = 80 MHz. The maximum insertion loss in the passband is 4 dB. The filter has a 50 Ω input and output impedance, SubMiniature version A connector termination, and significantly reduced form factor. The wide stopband frequency range and narrow passband in conjunction with small dimensions make the filter suitable to use it as a part of a high sensitive readout for superconducting quantum circuits, such as superconducting quantum bits and cryogenic parametric amplifiers.
We show an experimental study of superconducting X-mon qubit based on low noise wideband measurement setup. The setup includes wide stop-band low frequency filters with a stopband frequency up to 13 GHz, the broadband low-noise cryogenic microwave amplifier (cLNA) operating at 3.8 K. The obtained power dissipation of the cLNA is below 20 mW, the frequency operating ranges from 6 GHz to 12 GHz with a gain of 30 dB. The equivalent noise temperature of the amplifier is bellow 6 K for the presented frequency range. We demonstrate here the characterization of the superconducting X-mon qubit coupled to an on-chip coplanar waveguide resonator. We show standard qubit experiments: one and two-tones qubit spectroscopy, ac-Stark shift experiments for probing and excitation power sweeps.
Superconducting microelectronics (SME) technology has the potential of realizing very high speed digital receivers capable of performing direct digitization of radio frequency signals with very low power consumption. The SME receiver is implemented on a single chip using Niobium based low temperature superconductive (LTS) Josephson Junction (JJ) technology by HYPRES. Analogue RF filters are still required at the receiver front end and are key components of the overall superconductor digital receiver. SME receivers usually require two types of RF filters; a wideband bandpass filter and a bandstop filter (a notch filter). The notch filter is required to eliminate interference and unwanted signals in the passband. In this thesis, design of highly miniaturized lumped element wideband and bandstop filters is investigated and some challenges are addressed. The filters are fabricated by the HYPRES process and therefore can be integrated with the SME receiver on the same chip. \nIn a wideband filter, the coupling between the adjacent resonators is high. Achieving such a strong coupling is one of the challenges of designing wideband filters. The wideband filters realized with distributed elements usually suffer from very low spurious frequency. As the bandwidth of the filter becomes wider, the spurious peak of the second harmonic gets closer to the passband of the filter. In the first part of this work, the possibility of realizing lumped element superconducting bandpass filters (BPF) with a relative bandwidth of 80% is investigated. \nIn the second part of the thesis, design and realization of lumped element superconducting bandstop filters (BSF) is discussed. The challenge for designing a bandstop filter is providing a good match over a wide frequency range. So narrowband inverters cannot be used. Instead, usually λ/4 matched transmission lines provide 90° phase shift between the resonators of a notch filter. The possibility of replacing the long transmission line with other means or eliminating the inverters and using both shunt and series resonators are investigated. Having both series and shunt resonators introduces some new challenges that are addressed in the thesis and discussed thoroughly. \nA tunable notch resonator is presented. The tunability is provided by a superconducting MEMS varactor that is realized in our group by doing some post processing on the device fabricated by HYPRES. The tunability range of the device at cryogenic temperatures is investigated. A 3-pole tunable BSF is also designed that uses the same tunable resonators. The tunability of the filter is investigated through simulation.
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.