Abstract
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.
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