Thermalization of a flexible microwave stripline measured by a superconducting qubit

With the demand for scalable cryogenic microwave circuitry continuously rising, recently developed flexible microwave striplines offer the tantalyzing perspective of increasing the cabling density by an order of magnitude without thermally overloading the cryostat.
We use a circuit quantum electrodynamics setup to test the thermalization of input flex cables with integrated 60dB of attenuation distributed at various temperature stages.
From the measured decoherence rate of a superconducting fluxonium qubit, we estimate a residual population of the readout resonator below 0.007 photons and we measure a 0.22ms thermalization time for the flexible stripline attenuators.
Furthermore, we confirm that the qubit reaches an effective temperature of 26.4mK, close to the base temperature of the cryostat, practically the same as when using a conventional semi-rigid coaxial cable setup.