  Integration is currently the only feasible route towards scalable photonic
quantum processing devices that are sufficiently complex to be genuinely useful
in computing, metrology, and simulation. Embedded on-chip detection will be
critical to such devices. We demonstrate an integrated photon-number resolving
detector, operating in the telecom band at 1550 nm, employing an evanescently
coupled design that allows it to be placed at arbitrary locations within a
planar circuit. Up to 5 photons are resolved in the guided optical mode via
absorption from the evanescent field into a tungsten transition-edge sensor.
The detection efficiency is 7.2 \pm 0.5 %. The polarization sensitivity of the
detector is also demonstrated. Detailed modeling of device designs shows a
clear and feasible route to reaching high detection efficiencies.
