We present the general structure of two-photon S matrix for a waveguide coupled to a local quantum system that supports multiple ground states. The presence of the multiple ground states results in a non-commutative aspect of the system with respect to the exchange of the orders of photons. Consequently, the two-photon S matrix significantly differs from the standard form as described by the cluster decomposition principle in the quantum field theory.
PACS numbers:The scattering matrices (S matrices) are of essential importance for characterizing the interaction of quantum particles. On one hand, each element of a scattering matrix describes the probability amplitude of a particular scattering event. Thus every element of a scattering matrix is of direct experimental significance. On the other hand, from a theoretical point of view, the analytic structure of an S matrix is strongly constrained by symmetries and causalities, as well as by other general aspects such as the local nature of the interactions. Consequently, much of the literature on quantum field theory is devoted to the computation and elucidation of the structure of S matrices [1-4].Using the cluster decomposition principle [1,5,6], the standard form of two-particle S matrix listed in quantum field theory textbooks is S = S 0 + i T , where S 0 , the non-interacting part of the S matrix, is of the formand contains the product of two δ functions. The T matrix, which describes the interaction, is of the formand contains a single δ functions. Here, k 1,2 and p 1,2 are the momenta of the incident and outgoing particles, respectively. t k is the individual particle transmission amplitude and C p1p2k1k2 characterizes the strength of the interactions between two particles. Recently, this form is also shown to apply in waveguide quantum electrodynamics arXiv:1610.01727v1 [quant-ph]