John Burniston

We develop a flexible and robust framework for finite-size security proofs of quantum key distribution (QKD) protocols under coherent attacks, applicable to both fixed- and variable-length protocols. Our methods achieve high finite-size key rates across a broad class of protocols while imposing minimal requirements. In particular, it eliminates the need for restrictive assumptions such as limited repetition rates or the implementation of virtual tomography procedures. To achieve this goal, we introduce new numerical techniques for the evaluation of conditional sandwiched Renyi entropies, enabling tight key rate bounds without compromising generality. In doing so, we find an alternative formulation of the ``QKD cone'' studied in previous work, which may be of independent interest. Moreover, we illustrate the versatility of our framework by applying it to several practically relevant protocols, including decoy-state protocols. Furthermore, we extend the analysis to accommodate realistic device imperfections, such as independent intensity and phase imperfections. Overall, our framework provides both greater scope of applicability and better key rates than existing techniques, especially for small block sizes, offering a scalable path toward secure quantum communication under realistic conditions.

The security analysis of many protocols relies on closed form bounds on entropic quantities that model devices. These closed form expressions can typically only be found by exploiting some sort of symmetry not present in many realistic unstructured QKD protocols. Our software provides a framework for efficiently evaluating secret key rates of generic unstructured QKD protocols with tighter lower bounds while providing more flexible and realistic modelling capabilities. Through its modular structure, our software package breaks down the task of constructing a (numerical) security proof into well defined domains including protocol design, modelling implementations, security frameworks, and numerical optimization, each of which has its own community of experts. By utilizing modules built by these communities, we aim to facilitate wide spread collaboration throughout the QKD community. The newly expanded and redesigned software is expected to be released early May 2024.

The coherent one-way (COW) protocol is a promising commercial solution to practical quantum key distribution (QKD) due to its simple optical implementation. However, the non-IID structure of COW due to its inter-signal coherence makes standard security analysis inapplicable. Recently, it has been shown that a modified COW setup allows standard IID analysis, but at the cost of imposing extra limitations and increasing the number of pulses required for each bit. Here we propose a variant that possesses the IID structure and completely retains the optical setup of COW, but with a different data processing scheme that ignores inter-signal information. We obtain key rate lower bound close to analysis for the previously proposed IID variant, and achieves a higher number of key bits transmitted per second.

Without access to robust quantum memory or gates, long distance QKD relies upon trusted relays. Several implementations place these relays on satellites, however they are limited in computational power and numerically intensive tasks such as privacy amplification cause bottlenecks for continuous key exchange. Currently, one solution is the simplified trusted relay which leaves all privacy amplification to the end parties at a potentially significant cost to key rate. We developed a post processing technique called pre-privacy amplification which performs a small and efficient post processing step to boost key rates without any additional rounds of communication. For a simplified trusted relay running an asymptotic qubit six-state protocol, we demonstrate an increase to the maximum tolerable QBER from 9.05% to 11.7%. We also identify several sufficient conditions to determine functionally unique pre-privacy amplification maps, and connect it to the graph isomorphism problem.