  Quantum key distribution (QKD) has convincingly been proven compatible with
real life applications. Its wide-scale deployment in optical networks will
benefit from an optical platform that allows miniature devices capable of
encoding the necessarily complex signals at high rates and with low power
consumption. While photonic integration is the ideal route toward
miniaturisation, an efficient route to high-speed encoding of the quantum phase
states on chip is still missing. Consequently, current devices rely on bulky
and high power demanding phase modulation elements which hinder the
sought-after scalability and energy efficiency. Here we exploit a novel
approach to high-speed phase encoding and demonstrate a compact, scalable and
power efficient integrated quantum transmitter. We encode cryptographic keys
on-demand in high repetition rate pulse streams using injection-locking with
deterministic phase control at the seed laser. We demonstrate record
secure-key-rates under multi-protocol operation. Our modulator-free
transmitters enable the development of high-bit rate quantum communications
devices, which will be essential for the practical integration of quantum key
distribution in high connectivity networks.
