  Quantum key distribution (QKD) is the best candidate for securing
communications against attackers, who may in the future exploit
quantum-enhanced computational powers to break classical encryption. As such,
new challenges are arising from our need for large-scale deployment of QKD
systems. In a realistic scenario, transmitting and receiving devices from
different vendors should be able to communicate with each other without the
need for matching hardware. Therefore, practical deployment of QKD would
require hardware capable of adapting to different protocols and clock rates.
Here, we address this challenge by presenting a multi-rate, multi-protocol QKD
transmitter linked to a correspondingly adaptable QKD receiver. The flexibility
of the transmitter, achieved by optical injection locking, allows us to connect
it with two receivers with inherently different clock rates. Furthermore, we
demonstrate the multi-protocol operation of our transmitter, communicating with
receiving parties employing different decoding circuits.
