  Recent progress in integrated-optics technology has made photonics a
promising platform for quantum networks and quantum computation protocols.
Integrated optical circuits are characterized by small device footprints and
unrivalled intrinsic interferometric stability. Here, we take advantage of
femtosecond-laser-written waveguides' ability to process polarization-encoded
qubits and present the first implementation of a heralded controlled-NOT gate
on chip. We evaluate the gate performance in the computational basis and a
superposition basis, showing that the gate can create polarization entanglement
between two photons. Transmission through the integrated device is optimized
using thermally-expanded-core fibers and adiabatically reduced mode-field
diameters at the waveguide facets. This demonstration underlines the
feasibility of integrated quantum gates for all-optical quantum networks and
quantum repeaters.
