Fully modulator-free architecture for quantum key distribution in scalable quantum networks

Abstract Quantum key distribution (QKD) networks represent a foundational step toward the future quantum internet and have become a focus of intensive research. By analogy with the architecture of classical optical networks, we can observe that the high-capacity backbone typically relies on complex external modulation, while the cost-sensitive access layer prefers simpler, more compact, and more economical direct modulation. As the cornerstone of quantum networks, today’s QKD networks share this backbone-access hierarchy, yet their implementations remain heavily reliant on external modulators, leaving a clear ecological niche unfilled for directly modulated QKD in the access layer. Here, we propose and experimentally demonstrate the first fully modulator-free, full-degree-of-freedom direct-modulation QKD architecture that fills this niche. The transmitter combines optical injection locking with coherent optical interference to achieve simultaneous phase and intensity control without external modulators, enabling arbitrary quantum-state preparation while preserving inherent compatibility with standard time-bin-encoding receivers. Built with relatively inexpensive laser diodes and passive optical components, our experimental implementation achieves a secure key rate of 4.78 kbps at 30 dB channel loss, matching the performance of conventional modulator-based systems. By filling the long-standing direct-modulation niche in the QKD ecosystem, this work delivers a practical solution to the access-layer deployment challenge and opens a viable new direction for compact, scalable QKD implementation.