Koudia, Seid (2024) Quantum Coherent Control in Quantum Networks: From Point-to-Point Communications to Multipartite Entanglement Generation and Distribution. [Tesi di dottorato]

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Harnessing quantum phenomena in communication scenarios provides a fundamental advantage over classical communication protocols, from coding and decoding strategies to exotic phenomena that have no classical counterpart, like superadditivity of quantum channels capacities and nonlocality in quantum networks. From one side, a wide research interest has been devoted to understanding the impact of assisting quantum communications with genuinely quantum resources. A particular case that has been witnessing growing attention in the last decade is represented by quantum communications assisted by the genuinely quantum superposition of causal orders of channels, giving rise to what is known as causal activation of quantum channels capacities. In this thesis, we aim to pedantically study the differences and similarities between the three exotic phenomena, namely, superadditivity/superactivation and causal activation of channel capacities, in terms of fundamental quantum resources like entanglement and coherence. Moreover, we aim to investigate in depth the phenomenon of causal activation beyond the two causal orders of quantum operations for the aim of characterising the capacity bounds of the quantum switch in different setups of point-to-point communication. \\ From another side, huge efforts in the quantum computing community are devoted to building modular quantum architectures to beat the scalability bottlenecks of quantum chips, allowing for distributed quantum computing. Indeed, this is only possible if multipartite entanglement distribution achieves some efficiency thresholds. \\ Accordingly, a particular attention of this thesis is devoted to investigating the impact of superposition of causal orders of operations on more realistic scenarios of quantum networks beyond the point-to-point paradigm, where multipartite entanglement is needed. We study how graph states can be generated and distributed efficiently in quantum networks by harnessing different local coherent control strategies in the quantum network. Namely, different overhead bounds for the distribution of a target graph state are established. As an application, the design of a distributed stabilizer quantum error correcting code will be studied.

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