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IEEE International Symposium on Radio-Frequency Integration Technology (RFIT)

Characterization of Transmission Properties of Infrared Filters and Attenuators Up to 125 GHz

Junta Igarashi, Kosuke Mizuno, Ryo Ito, Shota Norimoto, T. Ishikawa, K. Inomata, Noriyoshi Hashimoto, Nobu‐Hisa Kaneko, Tomonori Arakawa

Published Aug 25, 2025• 0 citations
Publisher

Abstract

Recent advancements in quantum technology have driven a growing demand for microwave components capable of operating at cryogenic temperatures. Filtering frequencies above 80 GHz, corresponding to the superconducting gap in aluminum, is particularly important for superconducting qubits. Nevertheless, due to experimental difficulties, such evaluations have rarely been conducted, even at room temperature (RT). In this study, as an initial step, we characterize transmission properties of infrared (IR) filters and attenuators commonly used in low-temperature measurement setups at high frequencies up to 125 GHz at RT. This study offers insight into microwave components employed in quantum computing within the 100 GHz band.

DOI
10.1109/rfit65667.2025.11364584

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Recent advancements in quantum technology have driven a growing demand for microwave components capable of operating at cryogenic temperatures. Filtering frequencies above 80 GHz, corresponding to the superconducting gap in aluminum, is particularly important for superconducting qubits. Nevertheless, due to experimental difficulties, such evaluations have rarely been conducted, even at room temperature (RT). In this study, as an initial step, we characterize transmission properties of infrared (IR) filters and attenuators commonly used in low-temperature measurement setups at high frequencies up to 125 GHz at RT. This study offers insight into microwave components employed in quantum computing within the 100 GHz band.
Electromagnetic filtering is essential for the coherent control, operation and readout of superconducting quantum circuits at milliKelvin temperatures. The suppression of spurious modes around transition frequencies of a few GHz is well understood and mainly achieved by on-chip and package considerations. Noise photons of higher frequencies - beyond the pair-breaking energies - cause decoherence and require spectral engineering before reaching the packaged quantum chip. The external wires that pass into the refrigerator and go down to the quantum circuit provide a direct path for these photons. This article contains quantitative analysis and experimental data for the noise photon flux through coaxial, filtered wiring. The attenuation of the coaxial cable at room temperature and the noise photon flux estimates for typical wiring configurations are provided. Compact cryogenic microwave low-pass filters with CR-110 and Esorb-230 absorptive dielectric fillings are presented along with experimental data at room and cryogenic temperatures up to 70 GHz. Filter cut-off frequencies between 1 to 10 GHz are set by the filter length, and the roll-off is material dependent. The relative dielectric permittivity and magnetic permeability for the Esorb-230 material in the pair-breaking frequency range of 75 to 110 GHz are measured, and the filter properties in this frequency range are calculated. The estimated dramatic suppression of the noise photon flux due to the filter proves its usefulness for experiments with superconducting quantum systems.
Cryogenic filtering and attenuation play a crucial role in the control and readout of superconducting qubits, ensuring that unwanted noise does not degrade qubit coherence. These devices typically operate in the microwave C band (4–8 GHz) and use attenuators (input) and circulators/isolators (output) to control in-band noise, while reflective filters are used to reject out-of-band noise. However, these devices tend to become transparent to terahertz and infrared radiation. Such high-frequency radiation reaching the qubit destroys superconductivity and induces dissipation, degrading the coherence properties of these devices. Magnetically loaded epoxy (like Eccosorb TM^\mathrm{{TM}}TM ) has been widely used as a dielectric to construct filters that absorb high-frequency radiation while minimizing loss in the operating band. Here, we demonstrate the construction and characterization of absorptive filters with variable in-band attenuation by combining Eccosorb TM^\mathrm{{TM}}TM or iron powder mixed with Stycast in different ratios. We show that such filters can be made with sufficiently large in-band attenuation to replace the usual thin-film resistor-based attenuators and simplify cryogenic wiring by combining attenuation and IR filtering in a single device. We characterize the microwave performance at room temperature and at dilution fridge temperatures for devices with insertion losses ranging from 1 dB to 20 dB at 7 GHz. In a separate set of experiments, we also test the materials’ absorption capabilities at terahertz frequencies up to 450 GHz. Finally, we demonstrate high coherence in a 3D superconducting qubit ( T1,T2E≈100 μsT_1, T_2^E \approx 100~\mu sT1​,T2E​≈100 μs ) using these filters and attenuators, thereby confirming their efficacy.
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.
Superconducting qubits may suffer, despite extensive shielding, from interactions with residual noise photons arriving through signal lines to the device, which leads to qubit decoherence. To mitigate the photon population at the qubit frequency due to thermal radiation, attenuators can thermalize the electromagnetic environment at the sample stage. However, to prevent quasiparticle poisoning caused by Cooper pair breaking due to IR radiation, additional filtering is required. Such IR filters are routinely achieved using Eccosorb, a commercially available castable dielectric. We manufacture Eccosorb filters of various length and grade and characterize them at room and liquid nitrogen temperatures up to 67 GHz. Based on the obtained frequency dependent attenuation, we compute the residual noise photon population at base temperature and conclude on the preferred filter and attenuator configuration for superconducting qubit experiments. We validate the suggested microwave filtering for drive and flux bias lines by comparing coherent properties of transmon qubits for different filter configurations.
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.
The fragile quantum states of low-temperature quantum applications require protection from infrared radiation caused by higher-temperature stages or other sources. We propose a material system that can efficiently block radiation up to the optical range while transmitting photons at low gigahertz frequencies. It is based on the effect that incident photons are strongly scattered when their wavelength is comparable to the size of particles embedded in a weakly absorbing medium (Mie-scattering). The goal of this work is to tailor the absorption and transmission spectrum of an non-magnetic epoxy resin containing sapphire spheres by simulating its dependence on the size distribution. Additionally, we fabricate several material compositions, characterize them, as well as other materials, at optical, infrared, and gigahertz frequencies. In the infrared region (stop band) the attenuation of the Mie-scattering optimized material is high and comparable to that of other commonly used filter materials. At gigahertz frequencies (pass-band), the prototype filter exhibits a high transmission at millikelvin temperatures, with an insertion loss of less than 0.4 0.4\,0.4dB below 10 10\,10GHz.