Quantum noises in mesoscopic conductors and fundamental limits of quantum interference devices

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Abstract
We investigate quantum fluctuations of an electron-interference current in a perfectly fabricated, single-mode conductor. We first point out that the magnitude of the Johnson-Nyquist noise at zero temperature, when expressed as the fluctuation in the electron number, depends on the transmission coefficient only. We then clarify the physical origin of the ‘‘excess noise.’’ Discussions are also given for a more general case when an interference current is modulated by an external variables I, which may be either a classical or quantum-mechanical one. It is found that a possible fluctuation of I and the excess noise generate a new coupled noise. These results are used to discuss fundamental limits of quantum interference devices.