Engineering the spectral profile of photon pairs by using multi-stage nonlinear interferometers

Using the quantum interference of photon pairs in N-stage nonlinear interferometers (NLI), the contour of joint spectral function can be modified into islands pattern. We perform two series of experiments. One is that all the nonlinear fibers in pulse pumped NLI are identical; the other is that the lengths of N pieces nonlinear fibers are different. We not only demonstrate how the pattern of spectral function changes with the stage number N, but also characterize how the relative intensity of island peaks varies with N. The results, well agree with theoretical predictions in Ref. [1], reveal that the NLI with N pieces nonlinear fibers following binomial distribution can provide a better active filtering function. Our investigation shows that the active filtering effect of multi-stage NLI is a useful tool for efficiently engineering the factorable two-photon state - a desirable resource for quantum information processing.