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IEEE MTT-S International Microwave Symposium digest

Cryogenic Decade-Passband Superconducting Integrated Diplexer

Akim A. Babenko, Gregor Lasser, Nathan E. Flowers-Jacobs, Justus A. Brevik, Anna E. Fox, Zoya B. Popovic, Paul D. Dresselhaus

Published Jun 19, 2022• 3 citations
Publisher

Abstract

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.

DOI
10.1109/ims37962.2022.9865615

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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.
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 present the design and characterization of a broadband RF Josephson arbitrary waveform synthesizer (RF-JAWS) with a series array of 4500 Josephson junctions (JJs) and integrated low-pass/high-pass five-pole superconducting diplexers. The integrated diplexers enable broadband filtering of the feedthrough signal components in the drive-current pulses with decade-wide instantaneous bandwidth. The diplexers have at least 30 GHz passband with less than 0.8 dB insertion loss. The JJ array is driven with a delta-sigma pulse sequence that encodes 10 kHz and 1.005 GHz tones, and generates an open-circuit voltage of 22 mV rms (−26.18 dBm available power assuming a 50Ω\mathrm{\Omega }ΩThevenin equivalent source) at 1.005 GHz—a 25% increase compared to the state of the art. The drive current pulses undergo a three-step equalization to compensate for the linear distortion of the room-temperature electronics, the on-chip diplexers, and the remaining on- and off-chip components. The measured parasitic feedthrough voltages at 1.005 GHz are around −33 dBc, and their effect on the quantum locking ranges (QLRs) is quantified by measurements. The results demonstrated in this article show a significant step toward a broadband, integrated, quantum-based microwave voltage source with useful power above −30 dBm.
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.
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.
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.
Large-scale cryogenic quantum systems are constrained by an input-output bottleneck between room-temperature electronics and millikelvin stages, particularly in superconducting qubit platforms. This bottleneck is most acute for output lines, where bulky and expensive microwave components limit scalability. A promising approach for scalable characterization and testing is to perform signal multiplexing directly at the qubit plane. We demonstrate a cryogenic CMOS (cryoCMOS) RF multiplexer operating at 10 millikelvin with record-low static power consumption of 200 pW. The device provides < 2 dB insertion loss and > 30 dB isolation across DC-8 GHz. Direct connection to transmon qubits marginally affects coherence times in the range of 100 microseconds, enabling multiplexing of readout, flux and, in principle, XY drive lines. This work introduces cryoCMOS multiplexers as valuable tools for scalable, high-throughput cryogenic characterization and testing, and advances co-integrated quantum-classical control for future large-scale quantum processors.