  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 dB and 30 dB
attenuators are built on a quartz substrate and use 75 nm thick films of
nichrome for dissipative components and 0.001 mm 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 P_d in the attenuator. The minimum noise temperature T_n of the output
from the 20 dB attenuator was T_n less than and equal to 53 mK for no
additional applied power and T_n about 120 mK when dissipating 30 nW. In the
limit of large dissipated power (P_d > 1 nW) we find T_n proportional to
P_d^(1/5.4), consistent with detailed thermal modeling of heat flow in the
attenuators.
