# Continuous-variables quantum cryptography: asymptotic and finite-size security analysis

## TL;DR

This paper provides a comprehensive security analysis of continuous-variables quantum key distribution protocols, covering finite-size effects, measurement-device independence, asymmetric channels, and multiple users, advancing the understanding of practical quantum cryptography.

## Contribution

It introduces new finite-size security analysis methods and extends asymptotic security studies to asymmetric and multi-user scenarios without Gaussian approximations.

## Key findings

- Finite-size effects are quantified for two CV-QKD schemes.
- Security against coherent attacks is established for measurement-device-independent protocols.
- Asymmetric channel conditions and multi-user configurations are effectively analyzed.

## Abstract

In this thesis we study the finite-size analysis of two continuous-variables quantum key distribution schemes. The first one is the one-way protocol using Gaussian modulation of thermal states and the other is the measurement-device-independent protocol. To do so, we adopt an efficient channel parameter estimation method based on the assumption of the Gaussian variables and the central limit theorem introduced by Ruppert et al. [Phys. Rev. A 90, 062310 (2014)]. Furthermore, we present a composable security analysis of the measurement device independent protocol for coherent attacks with a channel parameter estimation that is not based on the central limit theorem. We also investigated, in the asymptotic regime, an asymmetric situation for the authenticated parties against the eavesdropper caused by fast-fading channels. Here we assume that the eavesdropper has the full control of the communication channel and can instantaneously change its transmissivity in every use of it. We assumed the simple model of a uniform fading and addressed the cases of one-way protocols, continuous-measurement device-independent protocol in symmetric configuration and its star network extension for three users. Finally, we extended the asymptotic study of the one-way protocols using an arbitrary number of phase-encoded coherent states assuming a thermal loss channel without using a Gaussian approximation.

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Source: https://tomesphere.com/paper/1906.05035