Robert Thew

We report the fabrication and characterization of 28-pixel P-SNSPD, reaching 88% system detection efficiency (SDE) at the single photon level. The detector is able to detect single-photon events at 250 Mcps with 50% nominal SDE, using only a single coaxial read-out cable, and maintains a timing jitter below 80 ps until 200 Mcps. Moreover,we achieve 1 Gcps detection rates by using only 4 P-SNSPD detectors and an 1:4 commercially available optical splitter Finally, we show how the P-SNSPD architecture allows us to maintain a very low jitter even at the high detection rates. We finally analyze the PNR capability of the array and measure efficiencies of 75% at 2-photon and 60% at 3-photon at 1550nm.

Superconducting-nanowire single-photon detectors (SNSPDs) have enabled the realization of several quantum optics technologies thanks to their high detection efficiency, low dark-counts, and fast recovery time. Here, we will present a 14-pixel SNSPD array with a maximum system detection efficiency (SDE) of 90% that remains above 80% up to 400 Mcps, and we demonstrate the ability to reach detection rates of 1.5 Gcps with an absolute SDE of 45%. Furthermore, we will explain how such device has been integrated in a QKD set-up and enabled high-speed QKD, with secret-key rates exceeding 60 Mbps over a distance of 10 km. Moreover when used in a QKD setup, the array can improve resilience against blinding attacks by monitoring the coincidence clicks between the pixels. Finally we will show that the detector is able to distinguish few-photon number states in an optical pulse with high fidelity, without posing strict limitations on the shape of the incoming light. We achieve a 2-photon fidelity of 74% and 57% for a 3-photon state, which represent state-of-the-art results for fibre-coupled SNSPDs. Such detectors could find immediate application in LOQC protocols where the capability to distinguish few photon-number states is sufficient – that is, either ‘1’ vs ‘more than 1 photons’.

There are two main Grand Challenges for academic research in quantum communication. The first one concerns "device independent QKD", that is an implementation of Quantum Key Distribution that exploits the nonlocal correlation observed in violations of Bell's inequality to realize "self testing QKD apparatuses". The second one aims at futuristic continental scale quantum networks. The latter requires, among others, multimode quantum memories with close to a second memory times, a fascinating challenge. Interestingly, quantum networks also lead us to a refreshing revisit of nonlocality.

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.

With the maturity of Quantum Technologies, namely Quantum Key Distribution (QKD) and Quantum Random Number Generation (QRNG), there has been mounting interest in scalable and inexpensive solutions for both academia and industry. To address the practicality and security requirements for QRNGs, we are developing a self-testing QRNG system based on homodyne detection with a fully integrated optical set-up. We use an Indium Phosphide (InP) photonic integrated circuit (PIC) with a high-speed 2.5GHz phase modulation that was designed and developed in collaboration with HHI Fraunhofer. All optical components are integrated in a 12×10 mm2 chip. It is then glued to a PCB designed in-house with electrical connections to the chip for full control and read-out of the results of the homodyne measurements. Another PCB, also designed in-house, is used to interface between the PIC and a field-programmable gate array (FPGA), which determines the quantum states to be prepared and reads out the homodyne detection. A graphics processing unit (GPU) connected to the FPGA then performs the statistical analysis of the data. The system operates at 1.25GHz and extraction rates above 18% are expected.