Sapphire sphere-based resin efficiently filters wide range of noise in superconducting circuits

Superconducting circuits such as quantum circuits require protection from incident optical or infrared radiation originating from leaks or higher-temperature stages in cryogenic setups. Filtering out this noise remains a challenge, however, due to the need to block a wide range of wavelengths, from nanometers to millimeters.

Griedel et al. have developed a material that efficiently filters out radiation across a wide range of wavelengths while transmitting photons at low gigahertz frequencies. They employed the concept of Mie scattering, which describes how homogeneous spheres scatter electromagnetic plane waves when wavelengths are comparable to the size of particles embedded in media, to craft a non-magnetic epoxy resin containing sapphire spheres. They then characterized several different material compositions with various sphere diameters and frequencies.

“We looked for a material that exhibits low attenuation in the microwave regime and tailored its behavior within the blocking range,” said author Hannes Rotzinger. “The compound interacts strongly with incident radiation when the wavelength of the photons is similar to the diameter of the sapphire spheres.”

The group found their filter significantly attenuated the infrared stop band while maintaining a high transmission for gigahertz frequencies in the pass band compared to other commonly used materials.

“This is described by Mie theory, which we used to calculate the distribution of sphere diameters in the compound,” said author Markus Griedel. “We made the distribution wide enough to ensure that optical and infrared radiation is strongly scattered or absorbed, while longer wavelength photons, such as microwaves, are not altered much.”

The group next looks to examine how to optimize for other parts of the low-temperature environment, such as connecting signal lines.

Source: “Low-loss material for infrared protection of cryogenic quantum applications,” by Markus Griedel, Maximilian Kristen, Biliana Gasharova, Yves-Laurent Mathis, Alexey V. Ustinov, and Hannes Rotzinger, Chaos (2026). The article can be accessed at https://doi.org/10.1063/5.0323074 .