Cryogenic microwave frequency filters beyond 20 GHz and their impact on superconducting quantum circuits*

The leakage of microwave photons in the coaxial wiring to a superconducting quantum circuit at frequencies above the superconducting gap of Aluminium (~ 82 GHz) can lead to a reduction in coherence due to quasiparticle generation. The work reports compact cryogenic microwave filters to attenuate higher frequency radiation before reaching superconducting circuits operating in the low GHz range at millikelvin temperatures. The cut-off frequencies between 1 to 10 GHz and the roll-off are set by the filter length, and the absorptive material used in the filter. These parameters are distinct for CR-110 and Esorb-230 tested in the work. The experimental characteristics of the filters are done at room and cryogenic temperatures and up to frequencies of 70 GHz. The calculated coaxial cable attenuation up to 600 GHz including TE and TM modes shows a significant noise photon occupation number at the frequencies above the Aluminium superconducting energy gap. The transmission of Esorb-230 filter material probed at 67 to 110 GHz shows strong attenuation relative to PTFE. The same material can also be used for the outer radiation shielding of the superconducting circuits.