Derivation of quantum master equation with counting fields by monitoring a probe

We show a microscopic derivation of a quantum master equation with counting terms which describes the electron statistics. A localized spin behaves as a probe whose precession angle monitors the net electron current by the magnetic moment interaction. The probe Hamiltonian is proportional to the current and is determined self-consistently for a model of a quantum dot. Then it turns out that the quantum master equation for the spin precession contains the counting terms. As an application, we show the fluctuation theorem for the electron current.