We measure photon-assisted tunneling in 4- and 6-junction electron pumps at photon frequencies up to 60 GHz. We determine the microwave voltage at the pumps using noise thermometry. The standard theory of leakage in the electron pump, modified to include photon-assisted tunneling, describes our experiments well. From this test of theory we argue that, in the absence of external microwaves, photon-assisted tunneling driven by $1/f$ noise is an important error mechanism in electron pumps.

• Received 15 September 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.5192