Rob Thew

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’.

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.