Noise of a single-electron transistor in the regime of large quantum fluctuations of island charge out of equilibrium

By using the drone-fermion representation and the Schwinger-Keldysh approach, we calculate the current noise and the charge noise for a single-electron transistor in the nonequilibrium state in the presence of large quantum fluctuation of island charge. Our result interpolates between those of the ``orthodox'' theory and the ``cotunneling theory.'' We find the following effects which are not treated by previous theories: (i) At zero temperature $T=0$ and at finite applied bias voltage $|\mathrm{eV}|\ensuremath{\gg}{T}_{\mathrm{K}},$ where ${T}_{\mathrm{K}}$ is the ``Kondo temperature,'' we find that the Fano factor is suppressed more than the suppression caused by Coulomb correlation both in the Coulomb blockade regime and in the sequential tunneling regime. (ii) For $T\ensuremath{\gg}|\mathrm{eV}|/2\ensuremath{\gg}{T}_{\mathrm{K}},$ the current noise in the presence of large charge fluctuation is modified and deviates from the prediction of the orthodox theory. However, the Fano factor coincides with that of the orthodox theory and is proportional to the temperature. (iii) For $eV,T\ensuremath{\lesssim}{T}_{\mathrm{K}},$ the charge noise is suppressed due to the renormalization of system parameters caused by quantum fluctuation of charge. We interpret it in terms of the modification of the ``unit'' for island charge.