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Scientific Reports

Magnetically loaded dielectric based absorptive filters and attenuators with tunable attenuation for superconducting quantum circuits

Srijita Das, Madhavi Chand, Samuel John, Snehal Haldankar, Meghan P. Patankar, Shriganesh Prabhu, R. Vijay

Published Jul 31, 2026• 0 citations
Publisher

Abstract

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 \(^\mathrm{{TM}}\) ) 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 \(^\mathrm{{TM}}\) 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 ( \(T_1, T_2^E \approx 100~\mu s\) ) using these filters and attenuators, thereby confirming their efficacy.

DOI
10.1038/s41598-026-63969-z

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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 (^\mathrm{{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 (^\mathrm{{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 ( (T_1, T_2^E \approx 100~\mu s) ) using these filters and attenuators, thereby confirming their efficacy.
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.
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.
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.
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 noise is one of the main external factors decreasing superconducting qubit coherence. Matched coaxial filters can prevent microwave and IR photons' negative influence on superconducting quantum circuits. In this report, we describe the design and fabrication process of matched coaxial filters for noise-sensitive measurements at millikelvin temperatures. A robust transmission coefficient and ultralow reflection loss of −20 dB in the frequency range up to 20 GHz is achieved. Fabricated low-pass filters have linear transmission and reflection characteristics with 3 dB-cutoff frequency of 1.5–2.5 GHz. A method for extracting the propagation constant and filter impedance from scattering parameter measurements is demonstrated. This method is experimentally approved on a filter with a compound of Cu powder and Stycast epoxy resin and a filter filled with ECCOSORB CR-110 epoxy resin. The proposed design and assembly technology are versatile for various compounds and provide highly repeatable geometric and microwave characteristics. Furthermore, we demonstrate that these low-pass coaxial filters can be effectively utilized to improve superconducting qubit relaxation due to suppressing standing waves originating from reflections in control coaxial cables.