Microwave attenuators for use with quantum devices below 100 mK

To reduce the level of thermally generated electrical noise transmitted to superconducting quantum devices operating at 20 mK, we have developed thin-film microwave power attenuators operating from 1 to 10 GHz. The 20 and 30 dB attenuators are built on a quartz substrate and use 75 nm thick films of nichrome for dissipative components and 1 μm thick silver films as hot electron heat sinks. The noise temperature of the attenuators was quantified by connecting the output to a 3D cavity containing a transmon qubit and extracting the dephasing rate of the qubit as a function of temperature and dissipated power $Pd$ in the attenuator. The minimum noise temperature $Tn$ of the output from the 20 dB attenuator was $Tn≤53$ mK for no additional applied power and $Tn≈120$ mK when dissipating 30 nW. In the limit of large dissipated power (⁠$Pd>1$ nW), we find $Tn∝Pd1/5.4$⁠, consistent with detailed thermal modeling of heat flow in the attenuators.