Metrological accuracy of the electron pump

We have operated a five-junction electron pump with an error for transferring electrons of approximately 0.5 part per ${10}^6$. The error predicted from existing theory is several orders of magnitude smaller, thus implying that our present understanding of the Coulomb blockade is incomplete. We conjecture that the errors arise from photon-assisted tunneling, where the photon energy is supplied by noise from the environment.