  Widespread commercial adoption of telecom-band quantum-key-distribution (QKD)
will require fully integrated, room-temperature transmitters. Implementing
highly efficient spontaneous parametric down-conversion (SPDC) on a platform
that offers co-integration of the pump laser has been an outstanding challenge.
Here, using such a platform based on AlGaAs-on-insulator waveguides, we report
telecom-band SPDC (and second harmonic generation) with exceedingly large
efficiencies of 26 GHz generated pairs/mW over a 7 THz bandwidth, which would
saturate the usable photon-flux for a 70-channel wavelength-multiplexed
QKD-system at merely 1.6 mW of pump laser power.
