Verification of finite size continuous-variable quantum key distribution under coherent attacks

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Quantum key distribution (QKD) aims at the distribution of secret information, only known, in the simplest case, to two directly involved parties usually called Alice and Bob. In contrast to classical cryptography, fundamental laws of quantum mechanics ensure that the information, encoded in quantum states, cannot be copied unnoticed. It is widely considered the most mature branch of quantum information science, where even some commercial implementations are available. The aim of this project is to build a quantum cryptography system with entangled laser beams, which improves on earlier work in several significant ways. We will for the first time establish a continuous variable (CV) QKD protocol providing security against most general, coherent attacks. Hence, in contrast to previous CV proposals and realizations, which assess security by an asymptotic formula in terms of the quantum error rate, the new protocol does not require strong additional assumptions constraining the type of attack. The first security proof without such constraints was only recently given by the theory partner of this project. We will exploit record levels of entanglement between twin laser beams, which are only available in the lab of the experimental partner. This is a prerequisite for achieving the demanding criteria set by the new security proof. The aforementioned asymptotic formula only asserts the existence of suitable data processing to generate a key. In most previous experiments the processing step was therefore often taken for granted. We will implement it, thus, generate actual key. The project extends earlier joint work of the two partners, in which a precursor of the planned twin laser beam source was shown to meet the asymptotic security criteria.