The current and the charge noise of a single-electron transistor in the regime of large charge fluctuations out of equilibrium

By using the Schwinger-Keldysh approach, we evaluate the current noise and the charge noise of the single-electron transistor (SET) in the regime of large charge fluctuations caused by large tunneling conductance. Our result interpolates between previous theories; the "orthodox" theory and the "co-tunneling theory". We find that the life-time broadening effect suppresses the Fano factor below the value estimated by the previous theories. We also show that the large tunnel conductance does not reduce the energy sensitivity so much. Our results demonstrate quantitatively that SET electrometer can be used as the high-sensitivity and high-speed device for quantum measurements.