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arXiv:1403.2909 · Review of Scientific Instruments

Impedance matched absorptive thermal blocking filters

Edward J. Wollack, David T. Chuss, Karwan Rostem, Kongpop U-yen

Published Mar 1, 2014• 14 citations
arXivHTMLPDF

Abstract

We have designed, fabricated, and characterized absorptive thermal blocking filters for cryogenic microwave applications. The transmission line filter's input characteristic impedance is designed to match 50 Ω and its response has been validated from 0 to 50 GHz. The observed return loss in the 0 to 20 GHz design band is greater than 20 dB and shows graceful degradation with frequency. Design considerations and equations are provided that enable this approach to be scaled and modified for use in other applications.

DOI
10.1063/1.4869038

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We have designed, fabricated, and characterized absorptive thermal blocking filters for cryogenic microwave applications. The transmission line filter's input characteristic impedance is designed to match 50 Ω and its response has been validated from 0 to 50 GHz. The observed return loss in the 0 to 20 GHz design band is greater than 20 dB and shows graceful degradation with frequency. Design considerations and equations are provided that enable this approach to be scaled and modified for use in other applications.
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 present an efficient fabrication method for absorptive microwave filters based on Eccosorb CR-124. Filters are fabricated from readily available parts, and their cutoff frequency can be set by their length. They exhibit desirable properties such as a very large and deep stop band with rejection beyond 120 dB at least up to 40 GHz, more than 10 dB return loss in both the pass and the stop band, and an error-function shaped step response without overshoot. Measurements at very low temperatures show that the filters thermalize on a time scale of approximately 100 s, and that they can absorb power as high as 100 nW with their noise temperature staying remarkably low, below 100 mK. These properties make the filters ideal for cryogenic filtering and filtering of intermediate frequency port signals of mixers.
We present a 4 : 1 superconducting transmission-line impedance transformer for cryogenic applications. The device transforms 25 Ω in the coplanar waveguide to 6.25 Ω in the microstrip and is designed to operate at 20 mK. Calibrated measurements in a dilution refrigerator demonstrate a -3 dB bandwidth from 1.6 to 13 GHz. In a modified Ruthroff design, a small capacitor is integrated at the input as a direct-current block, making it suitable for biasing and matching to low-impedance active circuits, such as superconducting quantum interference device (SQUID) 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.
Since superconducting quantum computers use microwave signals to control and readout their qubits, microwave transmission lines and circuits that can be used under cryogenic conditions are required. For microwave transmission lines, thermal insulation is essential to enhance wiring density and to prevent parasitic heating that can lead to increased computational errors. Thus, here we design and characterize a scalable, thermally insulated microwave transmission line. The transmission line designed for superconducting qubit readout at 5 GHz band achieves both thermal insulation and low loss, by employing resonator-based impedance matching in a microstripline structure. A flexible cable including eight planar transmission lines was designed, fabricated and characterized using S-parameter measurement, demonstrating low crosstalk below 40dB and signal attenuation of -23.5±0.07dB; this coincides with electromagnetic simulations. These microwave properties remained stable across a wide temperature range from room temperature down to 4 K, including steep gradients. The integration of chip attenuators further reduces thermal noise and protects sensitive cryogenic components. Our approach offers a promising solution for compact, high-density microwave wiring using flat cables under thermal gradients, necessary for large scale quantum computing.
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.