  Quantum key distribution (QKD) allows secret key exchange between two users
with unconditional security. For QKD to be widely deployed, low cost and
compactness are crucial requirements alongside high performance. Currently, the
majority of QKD systems demonstrated rely on bulk intensity and phase
modulators to generate optical pulses with precisely defined amplitude and
relative phase difference i.e., to encode information as signal states and
decoy states. However, these modulators are expensive and bulky, thereby
limiting the compactness of QKD systems. Here, we present and experimentally
demonstrate a novel optical transmitter design to overcome this disadvantage by
generating intensity- and phase-tunable pulses at GHz clock speeds. Our design
removes the need for bulk modulators by employing directly modulated lasers in
combination with optical injection locking and coherent interference. This
scheme is, therefore, well suited to miniaturization and photonic integration,
and we implement a proof-of-principle QKD demonstration to highlight potential
applications.
