Oral: A Cryogenic Testbed for Infrared Low-Pass Filters

Nonequilibrium quasiparticles produce decoherence and correlated errors in superconducting circuits. One of the main sources of quasiparticle poisoning is pair-breaking millimeter wave (mm-wave) and infrared (IR) radiation that travels down the microwave lines in the dilution refrigerator. Low-pass filters can be used to attenuate this radiation and effect. However, characterizing the filters' mm-wave performance at cryogenic temperatures is a significant challenge.

We propose a novel test setup to systematically characterize and compare the effectiveness of different mm-wave/IR low-pass filters. The setup comprises a radiator and a quasiparticle detector connected by coaxial cables, with the filter under test placed in between. The radiator sends a controllable power and spectrum of mm-wave and IR radiation down the cables. Any radiation which reaches the detector generates quasiparticles. By carefully designing a structure to house and connect the filter inline without changing the couplings between radiator and detector, we can reproducibly characterize the frequency-dependent attenuation that a filter provides, compare different filters to a control, and test the reproducibility of commercial filter performance. We discuss our progress using both blackbody and narrowband radiators, and using both nanobridge resonators and offset-charge-sensitive transmons as detectors.