Chao Wang

Given that most implementations of quantum cryptography systems require low light operations for security reasons, limiting the energy of incoming/outgoing optical signals is a central task. In this submission, we propose and demonstrate a novel and practical power limiter using the thermo-optical defocusing effect of an acrylic prism. The results show that a power limiting in the regime of mW or lower can be achieved, and at the same time possess desirable features like compactness, robustness, polarization and spectrum dimension independence, etc. Our work provides an effective way for limiting the incoming/outgoing optical energy, which is important for practical quantum cryptographic protocols. We believe it will attract much interest and possess the potential to become a standard tool for practical quantum applications.

Measurement-device-independent quantum key distribution (MDI-QKD) is the only known QKD scheme that can completely overcome the problem of detection side-channel attacks. Yet, despite its practical importance, there is no standard approach towards proving the security of MDI-QKD. Here, we present a simple numerical method that can efficiently compute almost-tight security bounds for any discretely modulated MDI-QKD protocol. To demonstrate the broad utility of our method, we use it to analyze the security of coherent-state MDI-QKD, decoy-state MDI-QKD with leaky sources, and a variant of twin-field QKD called phase-matching QKD. In all of the numerical simulations (using realistic detection models) we find that our method gives significantly higher secret key rates than those obtained with current security proof techniques. Interestingly, we also find that phase-matching QKD using only two coherent test states is enough to overcome the fundamental rate-distance limit of QKD. Taken together, these findings suggest that our security proof method enables a versatile, fast, and possibly optimal approach towards the security validation of practical MDI-QKD systems.

The power of quantum random number generation is more than just the ability to create truly random numbers. It can also enable self-testing, which allows the user to verify the implementation integrity of critical quantum components with minimal assumptions. In this work, we develop and implement a self-testing quantum random number generator (QRNG) chipset capable of generating 15.33 Mbits of certifiable randomness in each run, producing an expansion rate of 5.11×10-4 at a repetition rate of 10 MHz. The chip design is based on a highly loss-and-noise tolerant measurement-device-independent protocol, where random coherent states encoded using quadrature phase shift keying (QPSK) are used to self-test the quantum homodyne detection unit, well-known to be challenging to characterise in practice. Importantly, this proposal opens up the possibility to implement miniaturised self-testing QRNG devices at production scale using standard silicon photonics foundry platforms.

Quantum key distribution (QKD) provides information-theoretic security guaranteed by the laws of quantum mechanics, making it resistant to future computational threats, including quantum computers. While QKD technology shows great promise, its widespread adoption depends heavily on its usability and viability, with key rate performance and cost-effectiveness serving as critical evaluation metrics. In this work, we report an integrated silicon photonics-based QKD system that achieves a secret key rate of 1.213 Gbps over a metropolitan distance of 10 km with polarization multiplexing. Our contributions are twofold. First, in the quantum optical layer, we developed an on-chip quantum transmitter and an efficient quantum receiver that operate at 40 Gbaud/s at room temperature. Second, we designed a discrete-modulated continuous variable (DM CV) QKD implementation with efficient information reconciliation based on polar codes, enabling potentially high-throughput real-time data processing. Our results demonstrate a practical QKD solution that combines high performance with cost efficiency. We anticipate this research will pave the way for large-scale quantum secure networks.

Quantum key distribution (QKD) provides a practical method for distant parties to establish identical and secret keys. However, how quantum technologies can be practically used to protect user privacy with provable security remains an open question. Here, we report the first steps of our efforts to experimentally implement a symmetric private information retrieval (SPIR) scheme with QKD keys for fingerprint data retrieval. In the QKD layer, a three-user Measurement-device-independent QKD network is utilised for secure key distribution among the enquirer and data centres. In the application layer, an information-theoretically secure SPIR protocol is implemented to ensure both the privacy of the enquirer and the security of the database. Preliminary experimental results of the MDI QKD network implementation is presented, and simulations of the SPIR+QKD performance are also shown based on the experimental characterisation data.

Quantum random number generators (QRNGs) could generate numbers that are certifiably random even to a potential adversary who holds some side-information. However, many QRNGs require extremely precise characterisation of the source of the quantum states and the measurement apparatus. In this work, we propose a semi-device-independent QRNG protocol with untrusted homodyne detection. We show that our protocol is secure against quantum side-information, taking into account finite-size effects without making any assumption on the measurement device.

Characterizing the single-photon detection efficiency (SPDE) of a single-photon detector (SPD) is an essential but nontrivial task for various applications. Conventional methods require detailed detector models to calculate the estimated SPDE, which are not always available. In this work, a generalized method based on decoy-state for accurate characterization of SPDs is proposed and experimentally demonstrated. This work provides a new toolbox for rigorous SPD characterization with relaxed assumptions on the detector model, opening new possibilities in device calibration standards and quantum information applications.

By making reasonable assumptions on realistic systems, we propose and implement the first ultra-high-speed CHSH experiment working at 40GHz demonstrating a gigabit quantum certified random number throughput. Moreover, our scheme is suitable for optical chip design since it only requires standard optical components and balanced detectors. Furthermore, our scheme paves the way for the promising research direction to utilise noisy detectors for quantum system construction, which might be helpful for certain noise-sensitive applications, e.g. quantum sensing and quantum computing.