Hugo Zbinden

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’.

Bandwidth-limited devices in the transmitter of fast QKD implementations cause pulse correlations that leak information about previous setting choices. To take them into account in the existing security proofs, a measure of their strengths is needed. This is experimentally challenging, especially for long-range correlations, which are not experimentally accessible. In this work, we propose a new characterization method that exploits a linear model of the modulation devices. We show that this model predicts an upper bound for arbitrary order correlations that makes their characterization possible. We also present experimental results using the proposed method. In doing so, we can retrieve security even in the presence of arbitrary long correlations, with similar performance to classical security proofs.

Quantum key distribution (QKD) protocols leverage quantum mechanics to achieve information theoretically secure communication, yet real-world implementations must address experimental limitations, particularly phase correlations in weak coherent laser pulses (WCPs). High-speed gain-switching lasers, commonly used in QKD, can exhibit residual photons causing phase correlations between consecutive pulses, challenging the perfect phase randomization assumption crucial for the decoy-state BB84 protocol. Theoretical work has proposed security proofs that require knowledge of how closely each phase's probability distribution approximates uniformity, which is complex to estimate experimentally. In this study we introduce an experimental method to characterise phase correlations of any length under realistic conditions by modelling the phase generation process within the laser cavity. Additionally, we experimentally benchmark this practical routine for measuring second-order correlations using a double Michelson interferometer with tunable amplitude attenuators, allowing comprehensive characterisation of the phase generation process and accurate measurement of the phase probability distribution, thus enhancing the security of QKD systems.

Quantum key distribution (QKD) has raised as an attractive alternative to classical cryptography due to its security being provided by quantum mechanics rather than relying on algorithms that could potentially be broken in the future, rendering current communications insecure. However, many of the security proofs rely on assumptions that may not agree with reality, for instance, device imperfections can open loopholes that could potentially be exploited by a malicious party in order to extract part, if not all, of the secret key.

Quantum random number generators (QRNGs) have obtained notable attention and undergone substantial development, driven by their utility across diverse fields including simulations, gambling, and cryptography. This surge in interest stems from their unique capacity to deliver inherent randomness, which can only be derived from the probabilistic nature of quantum mechanics. The key challenge lies in validating the quantum origin of the randomness produced, which usually requires either a thorough characterization of the elements in the setup or very experimentally challenging loophole-free bell tests. In this work, we present a simple, self-testing and cost-effective quantum random number generator (QRNG) designed to operate with an untrusted measurement device and a partially characterized source, yielding a high rate of random bits. We consider a prepare-and-measure scenario where the preparation device takes a binary input x and a binary output b is received from the measurement device. Depending on the input, the preparation device sends either a weak coherent state (x=1) or a vacuum state (x=0). The measurement device employs homodyne detection to distinguish between these states, and the output value is chosen when the detector current is below (b=0) or above (b=1) a certain threshold. In order to certify the quantum origin of the randomness generated by output b, we need to track the correlations between input and output and the average energy per pulse must respect an upper bound. By using a continuous wave laser to seed the pulsed laser that generates the states, we avoid the need for expensive electro-optical modulators as used in https://arxiv.org/abs/2004.08307. With this scheme we achieve an extraction rate of certified quantum randomness of around 625kHz.

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.

The ideal Quantum random number generator (QRNG) is a black box which allows the users to test the quantum nature of the generated numbers. Producing a device which is close to this ideal is very demanding and will yield a low rate of random bits. Here we propose a simple setup which is self-testing on the detection part, meaning that only the source has to be characterized. We expect the implementation of this device to yield a random bit rate of around 10 Mpbs.

This research introduces a simplified variation of the time-based BB84 protocol, employing time-bin encoding and one decoy state. The proposed approach significantly simplifies the security analysis, enabling the identification of potential vulnerabilities by avoiding interference in the transmission of specific state combinations. This simplification reduces the reliance on finite key analysis and allows us to better characterize the source imperfections without much compromise on the secret key rate (SKR).