Low-loss material for infrared protection of cryogenic quantum applications

The fragile quantum states of low-temperature quantum applications require protection from infrared radiation caused by higher-temperature stages or other sources. In particular, signal lines have to be manufactured to prevent infrared photons entering through dielectric openings while maintaining low microwave loss. 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 (passband), the prototype filter exhibits a high transmission at millikelvin temperatures, with an insertion loss of less than 0.4 dB below 10 GHz.