Low-loss Material Achieves Infrared Protection for Cryogenic Quantum Applications at Gigahertz Frequencies

Protecting the delicate quantum states within cryogenic systems from unwanted infrared radiation is a significant challenge in modern physics. Markus Griedel, Max Kristen, and Biliana Gasharova, alongside colleagues from the Karlsruhe Institute of Technology, have investigated a novel material designed to address this issue. Their research focuses on a composite material , an epoxy resin embedded with sapphire spheres , which leverages Mie-scattering to efficiently block infrared photons while allowing gigahertz frequencies to pass through unimpeded. This carefully engineered approach promises to improve the performance and stability of sensitive quantum devices by minimising thermal noise and signal loss. The team’s fabricated prototypes demonstrate high infrared attenuation alongside remarkably low insertion loss at millikelvin temperatures, potentially offering a substantial advancement over existing filtering technologies.

The fragile quantum states essential for low-temperature quantum applications demand shielding from infrared radiation originating from warmer stages or external sources. Researchers are proposing a material system designed to efficiently block radiation up to the optical range, whilst simultaneously transmitting photons at low gigahertz frequencies. This approach leverages the principle of Mie-scattering, where incident photons are strongly scattered when their wavelength is comparable to the size of particles embedded within a weakly absorbing medium. The objective of this work is to precisely tailor the absorption and transmission spectrum of a non-magnetic epoxy resin incorporating sapphire spheres, by simulating its dependence on the size distribution of those spheres.

Sapphire Composites for Infrared Radiation Blocking

To mitigate interference from infrared radiation in sensitive low-temperature applications, the study pioneered a novel material system designed to block optical radiation while maintaining transparency to gigahertz frequencies. Researchers engineered a composite material consisting of sapphire spheres embedded within a non-magnetic epoxy resin, leveraging the principle of Mie-scattering to tailor the material’s absorption and transmission spectrum. The size distribution of the sapphire spheres was carefully controlled to maximize radiation attenuation in the infrared region, achieving performance comparable to commonly used filter materials. The team meticulously fabricated several material compositions and comprehensively characterised them across optical, infrared, and gigahertz frequencies.

Infrared measurements were conducted to identify the stop band, the range of frequencies effectively blocked by the material, and to quantify the attenuation achieved through optimised Mie-scattering. Simultaneously, gigahertz measurements at millikelvin temperatures assessed the filter’s transmission characteristics, revealing a high transmission with an insertion loss of less than 1 dB below 1GHz, demonstrating the material’s efficacy in preserving signal integrity at cryogenic temperatures. The experimental setup involved precise control over the size and concentration of sapphire spheres within the epoxy matrix. Sapphire spheres, sourced as alumina powder with a defined particle size distribution, were uniformly dispersed throughout the resin before curing.

Optical constants were determined using established methods and incorporated into Mie scattering simulations to predict and refine the material’s performance. Data processing employed the Savitzky-Golay filter to smooth measured transmission spectra, enhancing the accuracy of the analysis. Further refinement of the material’s properties was achieved through detailed Mie scattering simulations, utilising experimentally measured complex refractive indices up to 60μm. These simulations allowed the team to model the interaction of electromagnetic radiation with the composite material, optimising the sphere size distribution for maximum infrared attenuation. This combination of advanced material fabrication, precise characterisation, and rigorous theoretical modelling enabled the development of a high-performance filter crucial for sensitive low-temperature experiments.

Sapphire Composites Attenuate Infrared Radiation Effectively

Scientists achieved substantial infrared attenuation using a novel material system composed of sapphire spheres embedded within an epoxy resin. The research focused on tailoring the absorption and transmission spectrum of this composite material by carefully controlling the size distribution of the sapphire spheres. Experiments revealed that the material effectively blocks radiation up to the optical range while maintaining high transmission at low gigahertz frequencies, crucial for sensitive low-temperature applications. The team measured the extinction efficiency, Qext, for sapphire spheres ranging from 0.45μm to 700μm in diameter, finding that Qext remained approximately constant below 10μm.

Detailed calculations, based on Mie theory implemented in the MiePython software package, demonstrated that a 50μm sapphire sphere scatters incident radiation at 100μm isotropically. Furthermore, simulations showed that increasing the distribution of sphere sizes enhances the extinction bandwidth, a key factor in achieving broad-spectrum absorption. The total extinction coefficient, μext,tot, was calculated using a model incorporating sphere size, density, and weighting factors, providing a predictive tool for material design. Infrared transmission and absorption measurements, conducted between 1μm and 1000μm, confirmed the material’s high absorption capabilities.

The SP0.45-80 and SP0.45-700 composites exhibited near-complete absorption at wavelengths below 200μm, primarily due to the smaller spheres within the mixture. Data shows that absorption decreased in the far-infrared region, aligning with the expected Rayleigh scattering limit for wavelengths exceeding the largest grain size. Notably, the SP0.45-80 compound demonstrated higher absorption compared to SP0.45-700, a result attributed to the sphere size distribution. Tests prove that the prototype filter exhibits high transmission at millikelvin temperatures, with an insertion loss of less than 1dB below 1GHz.

Measurements of various materials, including PTFE, HDPE, and metal-loaded polymers, were conducted to establish a benchmark for infrared absorption. The team recorded that Eccosorb CR124 and metal powder composites displayed the highest absorption across the entire infrared spectrum, extending into the microwave regime. By applying the Beer-Lambert law to samples of varying thicknesses, scientists determined the wavelength-dependent extinction coefficient, μext, for each material, providing a comprehensive dataset for optimizing infrared shielding performance.

Sapphire-Epoxy Composite Boosts GHz Transmission Performance

This work demonstrates a novel material system for filtering infrared radiation while maintaining transparency to gigahertz frequencies, crucial for sensitive low-temperature applications. Researchers successfully combined epoxy resin with precisely sized sapphire spheres, leveraging Mie scattering to achieve high attenuation in the infrared spectrum, comparable to established materials, alongside minimal attenuation in the microwave range. Prototypes fabricated using this compound exhibited significantly improved transmission at gigahertz frequencies, approximately forty times greater than conventional filters like Eccosorb CR124, while maintaining comparable infrared blocking capabilities. The significance of this achievement lies in the potential to enhance the performance of delicate low-temperature experiments, particularly in quantum computing and astrophysics. By reducing unwanted thermal noise, the developed filter allows for more accurate and sensitive measurements. Future research will likely focus on refining fabrication techniques to further optimise performance and explore the material’s scalability for broader implementation in cryogenic systems.