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Secure quantum communication with untrusted and lossy devices

par Masini, Michele
Président du jury Massar, Serge
Promoteur Pironio, Stefano
Publication Non publié, 2024-10-07

Thèse de doctorat

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Résumé :

Quantum Key Distribution (QKD) enables provably secure communication, but its security depends on accurately characterising quantum devices. Since this is a challenging task, any inaccuracies in characterising the devices can pose security risks. Device Independent (DI) QKD protocols address this issue by making minimal assumptions about the devices used, and we refer to them as untrusted devices.This thesis begins with an introduction to key concepts in quantum information and quantum optics that are relevant to quantum communication with untrusted devices, along with examples of quantum cryptographic protocols and tasks that can be performed using these untrusted devices. We then explore the limitations of DI QKD protocols and other protocols that use untrusted quantum devices. We introduce a new mathematical framework to study the necessary conditions for untrusted devices in the presence of photon losses and other common types of noise in quantum communication protocols. We find that protocols using untrusted devices require high detection efficiencies, which refer to the ability of the experimental setup to reliably detect quantum states. This significantly limits the distance over which DI QKD can be implemented.We present frameworks for analysing the security of DI QKD and apply these methods to the most common protocols of this type. We introduce a general analytical framework applicable to the study of the security of DI QKD protocols with devices producing only two inputs and two outputs. Subsequently, we outline two numerical methods that can be used to analyse setups with any number of inputs and outputs.Finally, we propose a hybrid solution that combines DI QKD with standard QKD. This approach involves partially trusting one side of the communication protocol while not trusting the other side. It is particularly suitable for secure communication between a server and its clients. Our findings show that this hybrid technology can overcome the distance limitations of DI QKD and achieve security over distances of the same order as standard QKD protocols.