  Time-bin encoding is an attractive method for transmitting photonic qubits
over long distances with minimal decoherence. It allows a simple receiver for
quantum key distribution (QKD) that extracts a key by measuring time of arrival
of photons and detects eavesdropping by measuring interference of pulses in
different time bins. In the past, coherent pulses have been generated using a
CW laser and an intensity modulator. A greatly simplified transmitter is
proposed and demonstrated here that works by directly modulating the laser
diode. Coherence between pulses is maintained by a weak seed laser. The
modulator-free source creates time-bin encoded pulses with a high extinction
ratio (29.4 dB) and an interference visibility above 97 %. The resulting QKD
transmitter gives estimated secure key rates up to 4.57 Mbit/s, the highest yet
reported for coherent-one-way QKD, and can be programmed for all protocols
using weak coherent pulses.
