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Commuting operations factorise
(2023)arXivConsider two agents, Alice and Bob, each of whom takes a quantum input, operates on a shared quantum system $K$, and produces a quantum output. Alice and Bob's operations may commute, in the sense that the joint input-output behaviour is independent of the order in which they access $K$. Here we ask whether this commutation property implies that $K$ can be split into two factors on which Alice and Bob act separately. The question can be ...Working Paper -
The debate over QKD: A rebuttal to the NSA's objections
(2023)arXivA recent publication by the NSA assessing the usability of quantum cryptography has generated significant attention, concluding that this technology is not recommended for use. Here, we reply to this criticism and argue that some of the points raised are unjustified, whereas others are problematic now but can be expected to be resolved in the foreseeable future.Working Paper -
Generalised entropy accumulation
(2022)arXivConsider a sequential process in which each step outputs a system $A_i$ and updates a side information register $E$. We prove that if this process satisfies a natural "non-signalling" condition between past outputs and future side information, the min-entropy of the outputs $A_1, \dots, A_n$ conditioned on the side information $E$ at the end of the process can be bounded from below by a sum of von Neumann entropies associated with the ...Working Paper -
Thought experiments in a quantum computer
(2022)arXivWe introduce a software package that allows users to design and run simulations of thought experiments in quantum theory. In particular, it covers cases where several reasoning agents are modelled as quantum systems, such as Wigner's friend experiment. Users can customize the protocol of the experiment, the inner workings of agents (including a quantum circuit that models their reasoning process), the abstract logical system used (which ...Working Paper -
Quantum Advantage in Cryptography
(2022)arXivEver since its inception, cryptography has been caught in a vicious circle: Cryptographers keep inventing methods to hide information, and cryptanalysts break them, prompting cryptographers to invent even more sophisticated encryption schemes, and so on. But could it be that quantum information technology breaks this circle? At first sight, it looks as if it just lifts the competition between cryptographers and cryptanalysts to the next ...Working Paper -
Embedding cyclic causal structures in acyclic spacetimes: no-go results for process matrices
(2022)arXivCausality can be defined in terms of a space-time structure or based on information-theoretic structures, which correspond to very different notions of causation. When analysing physical experiments, these notions must be put together in a compatible manner. The process matrix framework describes quantum indefinite causal structures in the information-theoretic sense, but the physicality of such processes remains an open question. At the ...Working Paper -
Security of quantum key distribution from generalised entropy accumulation
(2022)arXivThe goal of quantum key distribution (QKD) is to establish a secure key between two parties connected by an insecure quantum channel. To use a QKD protocol in practice, one has to prove that it is secure against general attacks: even if an adversary performs a complicated attack involving all of the rounds of the protocol, they cannot gain useful information about the key. A much simpler task is to prove security against collective attacks, ...Working Paper -
The black hole information puzzle and the quantum de Finetti theorem
(2021)arXivThe black hole information puzzle arises from a discrepancy between conclusions drawn from general relativity and quantum theory about the nature of the radiation emitted by a black hole. According to Hawking's original argument, the radiation is thermal and its entropy thus increases monotonically as the black hole evaporates. Conversely, due to the reversibility of time evolution according to quantum theory, the radiation entropy should ...Working Paper -
Device-Independent Quantum Key Distribution
(2021)arXivCryptographic key exchange protocols traditionally rely on computational conjectures such as the hardness of prime factorisation to provide security against eavesdropping attacks. Remarkably, quantum key distribution protocols like the one proposed by Bennett and Brassard provide information-theoretic security against such attacks, a much stronger form of security unreachable by classical means. However, quantum protocols realised so far ...Working Paper -
Security in Quantum Cryptography
(2021)arXivQuantum cryptography exploits principles of quantum physics for the secure processing of information. A prominent example is secure communication, i.e., the task of transmitting confidential messages from one location to another. The cryptographic requirement here is that the transmitted messages remain inaccessible to anyone other than the designated recipients, even if the communication channel is untrusted. In classical cryptography, ...Working Paper