Inline detection and reconstruction of multiphoton quantum states

Integrated single-photon detectors open new possibilities for monitoring inside quantum photonic circuits. We present a concept for the inline measurement of spatially encoded multiphoton quantum states, while keeping the transmitted ones undisturbed. We theoretically establish that by recording photon correlations from optimally positioned detectors on top of coupled waveguides with detuned propagation constants, one can perform robust reconstruction of the N-photon density matrix describing amplitude, phase, coherence, and quantum entanglement. We report proof-of-principle experiments using classical light, which emulates the single-photon regime. Our method opens a pathway towards practical and fast inline quantum measurements for diverse applications in quantum photonics.