Ronald Freund

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.

We present experimental findings of a versatile quantum key distribution (QKD) system for diverse application scenarios such as long-distance, metropolitan, and last-mile/in-house links. This is enabled by the system’s dual-wavelength support, automatic initialization, stabilizing feedback loops, and modular design, which allows for usage of commercial detectors and encryptors.

We present a quantum key distribution (QKD) platform targeting mid-range fiber, free-space and hybrid links. With its interfaces for third-party post-processing, commercial key management, encryptors and QRNG, the modular and flexible system enables easy integration into existing telecom infrastructures. Recent experiments demonstrate its seamless operation over fiber and free-space links.

When wavelength multiplexing a large number of quantum communication channels and time-phase coding is used, it’s desired in terms of cost and complexity to minimize the number of interferometers. Here, we demonstrate that a visibility sufficiently high to enable QKD operation can be maintained with only a single interferometer at the receiver and without the need for additional high frequency phase modulation.