# Alejandro Pozas-Kerstjens was awarded an SNSF Postdoctoral Fellowship

### 24 Sep 2024

Alejandro Pozas-Kerstjens (UNIGE, N. Gisin's Group) was awarded an SNSF Postdoctoral Fellowship for the project: **Certification and cryptography in quantum networks.** The project will run from 01.09.2024 – 31.08.2026.

**Scientific abstract: **

This project has the goal of creating the first quantum cryptographic protocols tailored for quantum networks. The multipartite quantum cryptographic protocols that already exist rely on assumptions that do not match the reality of the implementations that we expect in the future, namely that the parties are arranged forming a network where quantum systems are distributed only among subsets of the parties. Inserting the network structure is known to lower the experimental requirements for certifying the presence of non-classical phenomena, and now it is the time to exploit these advantages in practice. Yet, in order to fully exploit quantum networks for cryptography, new fundamental understanding of their components must be developed. This project is divided in three work packages, that cover this development of fundamental notions, building new tools to guarantee the presence of quantum phenomena in networks, and the development of new cryptographic protocols.The first part of the project will be devoted to the study of the building blocks of quantum networks. These are the quantum states that are distributed, the network structure according to which they are distributed, and the measurements that are performed on them. While much is known about which quantum states are useful for quantum cryptography, much less is known of the other two. This project will build an understanding of the role of entangled measurements for establishing quantum correlations in networks, and create tools for obtaining guarantees of, both, the presence of entangled measurements in a network and of the network structure itself. The studies will be developed within the formalism of device-independent quantum information, which allows to work with untrusted devices.The second work package concerns certification of non-classical phenomena in networks considering them as a whole, in contrast to a collection of building blocks. This becomes a crucial matter, because the traditional notions of non-classicality only detect that some process in some part of the network is non-classical, but fail to capture complete quantum behavior. This project will find witnesses of strong forms of non-classicality, with a focus in loop and chain networks. The choice of these networks is motivated by the fact that they underlie the quantum repeaters that are used in practice to distribute quantum states over long distances. At this point it is when one can look for realizations where certifying non-classicality is easier, in terms of experimentally relevant requirements, than with the traditional assumptions. In this regard this project will look for examples of activation phenomena, namely, examples of quantum systems and measurements over them whose statistics can be simulated using classical systems, but such that using multiple copies arranged in a quantum network lead to results that cannot be reproduced by measurements on classical systems. At this point, a search to use the tools developed in certifying non-classicality and network structure in real experiments will be conducted as well.Armed with the tools and concepts developed in the previous work packages, the final part of the project will be devoted to the end goal of creating network-based quantum cryptographic protocols. Being the most ambitious work package and also the one with the highest potential impact, it will take a gradual approach. First the question of how to insert the realistic assumption of an underlying network structure affects the proofs of security of established quantum cryptographic protocols, and which new vectors of attack appear, will be analyzed. This will be the trampoline to develop a full-fledged framework to study cryptographic protocols in networks, that will lead to the first proposal of network-based quantum cryptographic protocols and associated security proofs.As a whole, the body of work to be developed in this research project will lead to a solid understanding of quantum correlations in networks, advances in the fundamental understanding of quantum measurements, and practical tools to analyze real-world protocols. These tools will be applicable to, both, the small-scale networks that are now being implemented in laboratories, and the large-scale networks that we will build in the future as part of the quantum internet.