Gianluca Boso

Integrating Quantum Key Distribution (QKD) into optical metropolitan networks is a crucial step toward bringing quantum technologies into existing telecommunication fiber infrastructures. However, current state-of-the-art solutions still face major challenges, including their sensitivity to classical noise, especially spontaneous Raman scattering, limited transmission distances, and performance degradation under heterogeneous network conditions. In this work, we present a new QKD system operating in the O-band at 1295.56 nm, where the effect of the main noise sources is significantly reduced. Combined with narrow spectral filtering at the receiver, without the need for active temperature control, the system shows strong robustness across a wide range of dense wavelength division multiplexing scenarios, making it a practical option for real-world deployment. We validate its performance through extensive testing and demonstrate stable key generation while coexisting with different classical traffic conditions, with total launch powers of up to 17 dBm. These results represent an important step forward for the integration of QKD into existing fiber networks, supporting the path toward secure quantum communications at scale.

Phase randomization is essential for the security of practical quantum key distribution (QKD) systems. Commonly, implementations rely on laser sources (either actively phase-randomized, or gain-switched). However, at high repetition rates these show correlations, which can ultimate compromise security and performance. We present a 1.25 GHz fully phase-randomized QKD source based on a super luminescent diode (SLED) operating in the C-band as a compact and cost-effective alternative. The source generates ∼ 100 ps optical pulses with 400 ps time-bin separation, compatible with high-speed time-bin encoding. Interferometric measurements demonstrate > 99% visibility between adjacent time bins, confirming strong first-order coherence within a qubit, while the spontaneous-emission-driven nature of the SLED ensures intrinsic pulse to pulse phase randomization. The broadband architecture further enables operation across multiple ITU channels, supporting wavelength-multiplexed QKD from a single emitter. This work establishes a scalable SLED-based platform for high-speed time-bin QKD systems.

Clock synchronization between the transmitter (Alice) and receiver (Bob) is essential for practical quantum key distribution (QKD) systems. In time-bin protocols, a frequency mismatch between the local clocks of Alice and Bob leads to a timing drift that broadens photon detection-time histograms, increasing the quantum bit error rate (QBER) if left uncompensated. In many practical systems this issue is mitigated by distributing a reference timing signal over a dedicated channel. However, this approach complicates the deployment of QKD in existing telecommunication networks, motivating synchronization methods that operate directly on photon detections from the quantum channel. Here, we present a lightweight synchronization algorithm for time-bin BB84 systems based on an analytical model of detection-time statistics under clock drift. The model describes how a constant frequency mismatch modifies start–stop histograms over a finite acquisition time, enabling direct estimation of the clock drift directly from detection events. This leads to a simple algorithm requiring only two consecutive histograms per update and compatible with standard hardware. We experimentally validate the method on commercial QKD systems. Laboratory tests over 100 km of fiber and under variable quantum channel attenuation demonstrate rapid convergence and low QBER, comparable to that obtained using a shared reference clock. We further demonstrate stable operation over 24 hours on a 16 km section of the Geneva Quantum Network.

This work reports on the implementation of a test sequence mode in a commercial QKD system, the methods and instrumentation designed to characterize the quantum states transmitted by such a system in TS mode, and the results obtained. These results are the first reported on a commercial QKD system operated in such a mode.