Ignatius William Primaatmaja

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.

Device-independent quantum key distribution (DIQKD) is the art of using untrusted devices to distribute secret keys in an unsecure network. It thus represents the ultimate form of cryptography, offering not only information-theoretic security against channel attacks, but also against attacks exploiting implementation loopholes~\cite{lydersen2010hacking}. At its heart, DIQKD utilises nonlocal correlations---detected and certified by a Bell inequality---to establish secret correlations between the users. In recent years, much progress has been made towards realising the first DIQKD experiments, but current proposals are just out of reach of today’s loophole-free Bell experiments. Here, in this work, we close the gap between the theory and practice of DIQKD with a simple variant of the original protocol based on the celebrated Clauser-Horne-Shimony-Holt (CHSH) Bell inequality. In using two randomly chosen key generating bases instead of one, we show that the noise tolerance of DIQKD can be significantly improved. In particular, the extended feasibility region now covers some of the most recent loophole-free CHSH experiments, hence indicating that the first realisation of DIQKD already lies within the range of these experiments.

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 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.

Characterising the input-output photon-number distribution of an unknown optical quantum channel is an important task especially in the case of quantum cryptography. In practice, this would require true photon number sources and photon-number-resolving detectors, but these technologies are still work-in-progress. In this work, we propose an efficient technique called double-decoy method which can provide relevant partial information of the input-output photon-number distribution, including the fraction of events in which the unknown quantum channel accepts and outputs a single photon. These detections correspond to events in which the transmitted single photon survives a basis-independent filter just before the measurement. We apply the double-decoy method to quantum key distribution (QKD) and show that it can substantially reduce the background noise and systematic error at the privacy amplification level, thereby improving the current secret key rate and achievable distance for standard QKD protocols. We also believe that several applications beyond cryptography will benefit from this technique.

The security of measurement device-independent quantum key distribution (MDI QKD) relies on a thorough characterization of one's optical source output, especially any noise in the state preparation process. Here, we provide an extension of the loss-tolerant protocol [Phys. Rev. A 90, 052314 (2014)], a leading proof technique for analyzing the security of QKD, to MDI QKD protocols that employ mixed signal states. We first reframe the core of the proof technique, noting its generalization to treat d-dimensional signal encodings. Concentrating on the qubit signal state case, we find that the mixed states can be interpreted as providing Alice and Bob with a virtual shield system they can employ to reduce Eve's knowledge of the secret key. We then introduce a simple semidefinite programming method for optimizing the virtual twisting operations they can perform on the shield system to yield a higher key rate, along with an example calculation of fundamentally achievable key rates in the case of random polarization modulation error.