Nathan Walk

Random numbers play a crucial role in information technology, particularly as digital communication capacity continues to expand. Consequently, the need for secure and high-rate random number generation has become increasingly urgent. While integrated photonics technology holds promise for mass-producing optoelectronic quantum random number generators (QRNGs), there remains a challenge in developing fast, robust, and scalable solutions suitable for industrial deployment. Addressing this challenge, we present a fast QRNG solution in this study, leveraging a photonic integrated circuit (PIC) directly embedded onto a versatile electronic platform. Designed to withstand real-world applications, our PIC is packaged to align with industrial electronic assembly lines. To rigorously assess scalability and stability, these generators underwent week-long periods of continuous GHz operation. Furthermore, a QRNG was integrated into a quantum key distribution system, where despite operating in an uncontrolled environment, minimal variations in physical randomness were observed over 38 days, as measured from 2.9 million histograms. Finally, we implemented a security model for the QRNGs, enabling rate adjustment to match the actual randomness content and demonstrating secure generation at 2 Gbit/s.

We report on an experimental demonstration of a recently proposed (n, n)-QSS (quantum secret sharing) protocol, which can be shown to be secure against participant attacks, using a four-photon GHZ state. Our work leverages the generation of high-quality and high-brightness non-linear single photon sources to achieve a secure key rate of 745 bits/sec in the asymptotic regime marking an important step toward scalable quantum-secure communication in networks.

Whilst the use of multi-partite entanglement is known to offer an advantage over bi-partite protocols in certain contexts, the quest to find practical advantage scenarios is ongoing and substantial difficulties in generalising some bi-partite security proofs remain. We present rigorous results that address both these challenges. First, we prove the security of a variant of the GHZ state based secret sharing protocol against general attacks, including participant attacks which break the security of the original GHZ state scheme. We then identify parameters for a performance advantage over realistic bottleneck networks in terms of extractable secret bits per network use. We show that whilst channel losses limit the advantage region to short distances over direct transmission networks, the addition of quantum repeaters unlocks the performance advantage of multi-partite entanglement over point-to-point approaches for long distance quantum cryptography.

Quantum repeaters have long been established to be essential for distributing entanglement over longdistances. Consequently, their experimental realization constitutes a core challenge of quantum communi-cation. However, there are numerous open questions about implementation details for realistic near-termexperimental setups. In order to assess the performance of realistic repeater protocols, here we presentReQuSim, a comprehensive Monte Carlo–based simulation platform for quantum repeaters that faithfullyincludes loss and models a wide range of imperfections such as memories with time-dependent noise. Ourplatform allows us to perform an analysis for quantum repeater setups and strategies that go far beyondknown analytical results: This refers to being able to both capture more realistic noise models and analyzemore complex repeater strategies. We present a number of findings centered around the combination ofstrategies for improving performance, such as entanglement purification and the use of multiple repeaterstations, and demonstrate that there exist complex relationships between them. We stress that numericaltools such as ours are essential to model complex quantum communication protocols aimed at contributingto the quantum Internet.

A remarkable aspect of quantum theory is that certain measurement outcomes are entirely unpredictable to all possible observers. Such quantum events can be harnessed to generate numbers whose randomness is asserted based upon the underlying physical processes. We formally introduce, design, and experimentally demonstrate an ultrafast optical quantum random number generator that uses a totally untrusted photonic source. While considering completely general quantum attacks and using dedicated FPGA hardware for post-processing, we certify and generate in real time random numbers at a rate of 8.05 Gb/s with a composable security parameter of 10^{−10}. Composable security is the most stringent and useful security paradigm because any given protocol remains secure even if arbitrarily combined with other instances of the same, or other, protocols, thereby allowing the generated randomness to be utilized for arbitrary applications in cryptography and beyond. This work achieves the fastest generation of composably secure quantum random numbers ever reported.

We demonstrate continuous variable Gaussian entanglement distillation using a measurement-based noiseless linear amplifier. Our scheme relies on a heterodyne detection scheme, followed by a classical non-deterministic post selection of the measurements. We demonstrate that we are able to extract a subset of data presenting a stronger entanglement than the original resource, and that we can recover entanglement after transmission loss.