  Measurement-device-independent quantum key distribution (MDI-QKD) is a
technique for quantum-secured communication that eliminates all detector
side-channels, although is currently limited by implementation complexity and
low secure key rates. Here, we introduce a simple and compact MDI-QKD system
design at gigahertz clock rates with enhanced resilience to laser fluctuations
- thus enabling free-running semiconductor laser sources to be employed without
spectral or phase feedback. This is achieved using direct laser modulation,
carefully exploiting gain-switching and injection-locking laser dynamics to
encode phase-modulated time-bin bits. Our design enables secure key rates that
improve upon the state of the art by an order of magnitude, up to 8 bps at 54
dB channel loss and 2 kbps in the finite-size regime for 30 dB channel loss.
This greatly simplified MDI-QKD system design and proof-of-principle
demonstration shows that MDI-QKD is a practical, high-performance solution for
future quantum communication networks.
