Carbon-Nanotube-Based Microwave Filters for Superconducting Qubit Measurements

Qubit measurements suffer from stray photons. Existing solution are not ideal because their performance is limited in temperature and frequency. Here, we report on the design, fabrication and characterization of two types of lossy transmission line filters. The characteristic impedance of both filters is nominally 50Ω and they are fabricated from multiwalled carbon nanotubes embedded in stainless-steel powder and Stycast. We show that the scattering parameters remain unchanged down to 10mK over a wide range of frequencies spanning from 10MHz to 50GHz. The cutoff frequency of the filter is easily tuned by varying the concentration of carbon nanotubes in the mixture. The small dimension of these filters together with their flat return loss over a wide band make them a good candidate for cryogenic applications including qubit measurements.