Quantum Langevin Dynamics of a Two-State System Coupled to a Bosonic Bath
T. Tsuzuki · Progress of Theoretical Physics · 1989
It is pointed out that the simultaneous inclusion of the reaction field in addition to the direct field of the bath is essentially important to describe the dynamics of the two-state system coupled to a bosonic bath. This is formulated in the framework of the quantum Langevin equation where the direct field of the bath is treated as the quantum mechanical random force. We study in detail the quantum-mechanical and statistical-mechanical notions of the reaction field as well as the direct field in the Ohm-Drude case of the spectral intensity of the bath. It is shown that the non-interacting blip approximation of Leggett et al. is equivalent to the approximation of the complete neglect of the reaction field. We propose a fundamental equation of the quantum Langevin dynamics, accompanied by a brief discussion about the feature of the equation.