Jonas Hilt

In this work, we address the challenge of dynamically rerouting quantum key distribution (QKD) links during live operation without the need for a system restart. Our novel resynchronization method, combined with a qubit-based clock frequency recovery algorithm, enables seamless rerouting of quantum channels in software-defined networks (SDNs). We validate our method with our 625 MHz real-time BB84 QKD system, using free-running cost-effective quartz oscillators and without an optical clock channel. The effectiveness of our method is demonstrated by the reliable system operation covering fiber length changes exceeding 100 km and sustaining channel interruptions of multiple minutes. We believe that our findings will significantly enhance the utility of QKD systems and simplify their flexible integration into existing and future telecom infrastructures, including optically switched SDNs.

We present iQSync, a novel clock offset recovery method for quantum key distribution. Our method is specifically tailored towards low-level hardware implementations, e.g. on FPGAs or microcontrollers, and requires only very little RAM and basic CPU instructions, like additions and bit-shifts. No floating-point operations, as is the case for FFT-based approaches, are needed. Offset revovery with iQSync typically only requires a few thousand iterations over a simple loop and evaluates with sublinear average-case computational complexity, improving on previous results with super-linear complexity. We implemented our method on our real-time QKD platform and demonstrate excellent agreement with theoretically derived success probabilities for channel attenuations exceeding 70 dB.