  Integrated single-photon detectors open new possibilities for monitoring
inside quantum photonic circuits. We present a concept for the in-line
measurement of spatially-encoded multi-photon 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 density matrix describing the amplitude, phase, coherence
and quantum entanglement. We report proof-of-principle experiments using
classical light, which emulates single-photon regime. Our method opens a
pathway towards practical and fast in-line quantum measurements for diverse
applications in quantum photonics.
