  Improvement in secure transmission of information is an urgent practical need
for governments, corporations and individuals. Quantum key distribution (QKD)
promises security based on the laws of physics and has rapidly grown from
proof-of-concept to robust demonstrations and even deployment of commercial
systems. Despite these advances, QKD has not been widely adopted, and practical
large-scale deployment will likely require integrated chip-based devices for
improved performance, miniaturisation and enhanced functionality, fully
integrated into classical communication networks. Here we report low error
rate, GHz clocked QKD operation of an InP transmitter chip and a SiO$_x$N$_y$
receiver chip --- monolithically integrated devices that use state-of-the-art
components and manufacturing processes from the telecom industry. We use the
reconfigurability of these devices to demonstrate three important QKD protocols
--- BB84, Coherent One Way (COW) and Differential Phase Shift (DPS) --- with
performance comparable to state-of-the-art. These devices, when combined with
integrated single photon detectors, satisfy the requirements at each of the
levels of future QKD networks --- from point-of-use through to backbone --- and
open the way to operation in existing and emerging classical communication
networks.
