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Covariance Functions for Quantum Linear System Driven by Few Photons

In this paper, we consider that a quantum linear system is driven by two input channels, which contain a single-photon state and an unfactorizable two-photon state. Quantum entanglement between the two channels is analyzed by deriving the explicit forms of the input and output covariance functions. Simulations are conducted by tunning the coupling strengths between the two input channels and the quantum system, which has certain guiding effect on the design and application of quantum optical experiments.