Low-loss Material for Infrared Protection of Cryogenic Quantum Applications

TL;DR

The study tackles protecting cryogenic quantum devices from infrared radiation while preserving microwave pass-band performance. It proposes a non-magnetic dielectric composite of sapphire spheres in epoxy and uses Mie-scattering theory to tailor an infrared stop-band while maintaining low loss in the gigahertz range. Key results show infrared extinction of about $\mu_{ext} \approx 2~/\mathrm{mm}$ up to far-IR and pass-band extinction around $\mu_{ext} \approx 4\times 10^{-4}~/\mathrm{mm}$ below 10 GHz, with a prototype filter achieving insertion loss $<0.4$ dB for $f<10$ GHz at millikelvin temperatures and roughly 40× higher pass-band transmission than Eccosorb CR124. This approach offers a practical, low-loss infrared shield compatible with millikelvin quantum systems, enabling improved coherence by suppressing infrared-induced quasiparticle generation.

Abstract

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 $0.4\,$dB below $10\,$GHz.

Figures (11)

• Figure 1: (a) Scattering at a single sphere ($n=3.69$; $x=1.5$) in a epoxy resin matrix ($n_{env}=1.5$). (b) Spheres of different sizes in a epoxy matrix with scattering path for different wavelength from short (blue) to long (red) wavelength. (c) Solid colored lines represent Planck radiation spectrum of the different cryogenic stages (right axis, $L_f$); dashed lines indicate the superconducting gap frequencies for Niobium (black) and Aluminum (gray). The gray (black) line shows the desired optimal cut-off frequency. The visible light spectrum is indicated on the far left. (d) Calculated Mie scattering for sapphire spheres of varying sizes in an epoxy resin matrix as a function of wavelength (left axis). The black solid (dashed) line is the total extinction efficiency of the composition SP0.45-700 (SP0.45-80) of different sphere sizes (right axis).
• Figure 2: Absorption spectra of various materials at a thickness of 1.5mm. Please note the different scales. (a) Sapphire powder mixtures SP0.45-80 and SP0.45-700, and epoxy resin. (b) Single-diameter sapphire powder samples. (c) PTFE, HDPE (transparent and black), Epoxy UHU+ Endfest 300 and Stycast 2850FT. (d) Eccosorb CR124 and epoxy resin mixed with metal powders (copper and stainless steel).
• Figure 3: (a) Transmission as a function of thickness at 864µm for the SP mixtures, epoxy resin, and Eccosorb CR124. The solid line is a fit based on the Beer–Lambert law. (b,c) VNA transmission and reflection measurements of an filter implementation based on the SP0.45-700 compound. The insets show the schematics (r$_i=0.4$ mm, r$_s=2.2$ mm, $l=8$ mm). Red lines represent the measurements at room temperature (RT), blue at 15 mK.
• Figure 4: The extinction coefficients of SP, epoxy resin, and Eccosorb CR124 are shown as a function of wavelength. The left graph shows the infrared range (gray region indicates the exemplary fit at 864µm in Fig. \ref{['fig:microwave_spectroscopy_filter']}(a)). The right graph shows microwave data of the SP compound based filter shown in Fig. \ref{['fig:microwave_spectroscopy_filter']}(b,c) as well as for copper and stainless steel. The Eccosorb CR124 data are taken from Ref. lairdEccosorbrMFDatasheetEccosorb2015. Note the different y-scale on the left and right graph.
• Figure S1: left: Microscope picture of the powder mixture SP0.45-80. right: 8 mm copper microwave sample with SMA connectors, variety of SP0.45-700 insets.