Comment on "Generalized James' Effective Hamiltonian Method"

Abstract

In the paper carried out by Wenjun et al. \cite{Shao2017}, a generalization of the James effective dynamics theory based on a first version of the James method was presented. This however, is not a very rigorous way of deriving the effective third-order expansion for an interaction Hamiltonian with harmonic time-dependence. In fact, here we show that the third-order Hamiltonian obtained in \cite{Shao2017} is not Hermitian for general situations when we consider time-dependence. Its non-Hermitian nature arises from the foundation of the theory itself. In this comment paper, the most general expression of the effective Hamiltonian expanded up to third order is obtained. Our derived effective Hamiltonian is Hermitian even in situations where we have time-dependence.