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We have measured the current-voltage characteristic of a small capacitance tunnel junction coupled to a transmission line resonator. We calibrate the resonator using the sharp resonances displayed by the junction in the superconducting state, which corresponds to the pumping of the modes of the resonator by the ac Josephson current. With this calibration, we explain quantitatively the nonlinearity of the junction characteristic in the normal state as being due to the process by which a single electron tunnels by emitting a photon, the basic process of the theory of the effect of the electromagnetic environment on tunneling.
Holst et al. (Mon,) studied this question.