The purpose of this paper is to study the dynamics of a quantum coherent
feedback network composed of two two-level systems (qubits) driven by two
counter-propagating photons, one in each input channel. The coherent feedback
network enhances the nonlinear photon-photon interaction inside the feedback
loop. By means of quantum stochastic calculus and the input-output framework,
the analytic form of the steady-state output two-photon state is derived. Based
on the analytic form, the applications on the Hong-Ou-Mandel (HOM)
interferometer and marginally stable single-photon devices using this coherent
feedback structure have been demonstrated. The difference between
continuous-mode and single-mode few-photon states is demonstrated.Comment: 15 pages, 4 figures; accepted by Automatica; comments are welcome