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Cumulant generating function formula of heat transfer in ballistic system with lead-lead coupling

Based on two-time observation protocol, we consider heat transfer in a given time interval $t_{M}$ in lead-junction-lead system taking coupling between the leads into account. In view of the two-time observation, consistency conditions are carefully verified in our specific family of quantum histories. Furthermore, its implication is briefly explored. Then using nonequilibrium Green's function (NEGF) method, we obtain an exact formula for the cumulant generating function for heat transfer between the two leads valid in both transient and steady-state regimes. Also, a compact formula for the cumulant generating function in the long-time limit is derived, for which the Gallavotti-Cohen (GC) fluctuation symmetry is explicitly verified. In addition, a brief discussion of Di Ventra's trick regarding whether the effect of the repartitioning procedure of the total Hamiltonian on nonequilibium steady-state current fluctuation exists is given. All kinds of properties of nonequilibrium current fluctuations such as the fluctuation theorem in different time regimes could be readily given according to these exact formulas.