This thesis takes a deep dive into quantum resources and their applications for quantum networks. In particular, this work reviews the concept of quantum correlations beyond entanglement and introduces a method for deterministically creating them via local operations by using classical correlations as a resource. The structure of these states is analysed in multipartite quantum networks, highlighting their generally asymmetric nature. We then explore how these quantum correlations can be used for entanglement generation protocols themselves and compare them to directly distributing a maximally entangled state through noisy channels. We find that, in certain noise regimes, these protocols can indeed outperform direct entanglement distribution, provided the memories at each node are sufficiently maintained. The application of multipartite quantum teleportation to multiple receivers in a network is also assessed, exploring the necessary and sufficient resources to ensure that the performance of quantum telecloning surpasses that of all known methods involving only classical communication. Our results show that, despite the genuine multipartite entanglement present in the resource state, only a few bipartite entanglement pairings are necessary for such a feat. This allows for a potential reduction in resource consumption for future experimental implementations. Thesis is embargoed until 31 December 2026.
This thesis takes a deep dive into quantum resources and their applications for quantum networks. In particular, this work reviews the concept of quantum correlations beyond entanglement and introduces a method for deterministically creating them via local operations by using classical correlations as a resource. The structure of these states is analysed in multipartite quantum networks, highlighting their generally asymmetric nature. We then explore how these quantum correlations can be used for entanglement generation protocols themselves and compare them to directly distributing a maximally entangled state through noisy channels. We find that, in certain noise regimes, these protocols can indeed outperform direct entanglement distribution, provided the memories at each node are sufficiently maintained. The application of multipartite quantum teleportation to multiple receivers in a network is also assessed, exploring the necessary and sufficient resources to ensure that the performance of quantum telecloning surpasses that of all known methods involving only classical communication. Our results show that, despite the genuine multipartite entanglement present in the resource state, only a few bipartite entanglement pairings are necessary for such a feat. This allows for a potential reduction in resource consumption for future experimental implementations. Thesis is embargoed until 31 December 2026.
Quantum information science is an interdisciplinary field that investigates the principles and applications of quantum mechanics in information processing. Upcoming quantum technologies promise a range of valuable applications, including powerful simulation of complex systems, enhanced computational capabilities, high-precision sensing and metrology, and secure communication via quantum key distribution. A central concept is entanglement, a unique type of correlation between quantum systems that has no classical counterpart. Entanglement plays a pivotal role in many quantum protocols, particularly as a key resource in quantum communication, where protocols such as distributed quantum computation and secret sharing are executed in a distributed setting. Hence, distributing entanglement over long distances and manipulating it locally is essential for enabling quantum communication applications. However, the effectiveness of such applications is limited by the challenges of transmitting quantum information and maintaining precise control over quantum systems. For example, quantum information carriers, such as photons traveling through optical fibers, may be lost, or the operations performed may not be perfect. Consequently, it is crucial to develop robust tools for distributing and manipulating entanglement in distributed settings in order to fully unlock the capabilities of quantum technologies. In this thesis, we explore the influence of noise on the manipulation of entanglement in entanglement-based quantum networks. First, we examine the noise resilience of various resource states, i.e., pre-established entangled states, used to achieve bipartite or tripartite communication. This is followed by an analysis of a fully asymmetric network scenario, incorporating the noise introduced by the delays from classical messaging within the quantum network. We then propose classical communication protocols to achieve bipartite communication and evaluate their noise impact under the optimization of the arrangement of the pre-established entanglement. In addition, we propose an alternative approach to quantum repeaters, a key device for entanglement distribution, based on growing multipartite states, allowing entanglement to be recycled and patched. All investigations have been carried out employing a novel formalism, also developed in this thesis, for efficiently and accurately simulating noisy quantum processes on classical hardware, with broader applicability beyond quantum communication.
A quantum internet aims at harnessing networked quantum technologies, namely by distributing bipartite entanglement between distant nodes. However, multipartite entanglement between the nodes may empower the quantum internet for additional or better applications for communications, sensing, and computation. In this work, we present an algorithm for generating multipartite entanglement between different nodes of a quantum network with noisy quantum repeaters and imperfect quantum memories, where the links are entangled pairs. Our algorithm is optimal for GHZ states with 3 qubits, maximising simultaneously the final state fidelity and the rate of entanglement distribution. Furthermore, we determine the conditions yielding this simultaneous optimality for GHZ states with a higher number of qubits, and for other types of multipartite entanglement. Our algorithm is general also in the sense that it can optimise simultaneously arbitrary parameters. This work opens the way to optimally generate multipartite quantum correlations over noisy quantum networks, an important resource for distributed quantum technologies.
Quantum entanglement plays a central role in many quantum communication protocols. It allows distant particles to share correlations beyond the limits of classical interactions. Entanglement is essential for superdense coding, quantum teleportation, and secure cryptographic key distribution in quantum communication. It is also a pillar for developing quantum networks, where the management and distribution of entanglement are crucial for connecting distant nodes. Although perfect entanglement is a sought-after ideal, experimental imperatives, including entanglement distribution over long distances, often limit the quality of entangled states. An important question is whether weaker entanglement still offers advantages. First, we study more general tasks than dense coding to show that simpler measurements, combined with entanglement, allow advantageous and sometimes even optimal qubit communication protocols to be obtained. We also demonstrate that simple measurements can generate quantum correlations that cannot be modelled by two classical communication bits and can constitute an optimal protocol with quantum resources. We implement a novel Bell-type inequality tailored for certifying full network non-locality (FNN) to develop certification methods guaranteeing security on an entanglement-based network. Our experiment uses two pairs of polarised entangled photons in a network configuration with three nodes, briefly referred to as a bilocal network scenario. Finally, we show that weakly entangled states can improve communication over a qubit channel using only separate (local) measurements on isotropic non-steerable two-qubit states, without interference, of individual photons, across two communication tasks: secret sharing and its stochastic variant.
Quantum networks form the bedrock of emerging quantum technologies. By connecting quantum systems through entangled links, they enable remarkable applications such as distributed quantum computing, secure quantum communication, and entanglement-enhanced sensing. This thesis investigates several key challenges in quantum networks, namely resource distribution, percolation, and topology. The first part of the thesis analyses the limitations of entanglement distribution using separable states (EDSS) in the zero-added-loss photon multiplexing (ZALM) architecture. The study determines that EDSS protocols outperform direct entanglement distribution when only the separable carrier is sent through a noisy channel. However, when noise is considered in channels via transmission to the memories, this advantage is lost. The second part of the thesis investigates link-level state generation in a distillable multiplexed quantum repeater architecture. Three different photon encodings are compared for two distinct distillation protocols, and the results demonstrate that for the low-loss regime, the GKP photon encoding yields a higher distillable entanglement compared than both Single Rail and Dual Rail photon encodings, regardless of the distillation protocol. The final part of the thesis investigates the performance of an optimised genetic algorithm that infers the topology of an unknown quantum network. Operating in the framework of continuous-time quantum walks and using an externally attached sink probe, the algorithm can successfully reconstruct network topologies of up to 10 nodes. Moreover, when the number of probe sites is increased, the reconstruction process is significantly simplified, and the algorithm is capable of reliably reconstructing network topologies of up to 14 nodes.
Scaling the number of entangled nodes in a quantum network is a challenge with significant implications for quantum computing, clock synchronisation, secure communications, and quantum sensing. In a quantum network, photons interact with matter qubits at different nodes, flexibly enabling the creation of remote entanglement between them. Multipartite entanglement among multiple nodes will be crucial for many proposed quantum network applications, including quantum computational tasks and quantum metrology. To date, experimental efforts have primarily focused on generating bipartite entanglement between nodes, which is widely regarded as the fundamental quantum resource for quantum networks. However, relying exclusively on bipartite entanglement to form more complex multipartite entanglement introduces several challenges. These include the need for ancillary qubits, extensive local entangling operations which increases the preparation latency, and increasingly stringent requirements on coherence times as the number of nodes grows. Here, we analyse various schemes that achieve multipartite entanglement between nodes in a single step, bypassing the need for multiple rounds of bipartite entanglement. We demonstrate that different schemes can produce distinct multipartite entangled states, with varying fidelity and generation rates. Additionally, we discuss the applicability of these schemes across different experimental platforms, highlighting their primary advantages and disadvantages.
Quantum networks connect systems at separate locations via quantum links, enabling a wide range of quantum information tasks between distant parties. Large-scale networks have the potential to enable global secure communication, distributed quantum computation, enhanced clock synchronization, and high-precision multiparameter metrology. For the optimal development of these technologies, however, it is essential to identify the necessary resources and sub-routines that will lead to the quantum advantage, but this is demanding even for the simplest protocols in quantum information processing. Here we show that quantum networks relying on the long-distance distribution of bipartite entanglement, combined with local operations and shared randomness, cannot achieve a relevant quantum advantage. Specifically, we prove that these networks do not help in preparing resourceful quantum states such as Greenberger-Horne-Zeilinger states or cluster states, despite the free availability of long-distance entanglement. At an abstract level, our work points towards a fundamental difference between bipartite and multipartite entanglement. From a practical perspective, our results highlight the need for classical communication combined with quantum memories to fully harness the power of quantum networks.