Spatio-Spectral Quantum State Estimation of Photon Pairs from Optical Fiber Using Stimulated Emission

Developing a multi-dimensional quantum light source that carries more than one bit per photon is crucial for expanding quantum information applications. However, characterizing such a source, especially when there are more than one degrees of freedom involved, at the single-photon level is challenging due to the large parameter space as well as limited emission rates and detection efficiencies. Here, we develop and characterize a fiber-based photon-pair source that produces photon pairs correlated in both spatial and spectral modes. We use stimulated emission to comprehensively characterize the photon pairs which we show is more efficient than spontaneous or conventional techniques: it allows for real-time imaging of one of the photon pairs (stimulated signal), estimation of photon-pair quantum state without conducting a full quantum state tomography (5 vs. 36 measurements). We also experimentally reveal a previously overlooked factor that affects the possible spatial-mode entanglement generation in a fiber photon-pair source: dispersion in parity birefringence. This method may be useful in the optimization of various photon-pair source platforms in which complicated correlations across multiple degrees of freedom may be present.