The development of a future, global quantum communication network (or quantum internet) will enable high rate private communication and entanglement distribution over very long distances. However, the large-scale performance of ground-based quantum networks (which employ photons as information carriers through optical-fibres) is fundamentally limited by the fibre quality and link length. While these fundamental limits are well established for arbitrary network architectures, the question of how to best design these global architectures remains open. In this work, we take a step forward in addressing this problem by modelling global quantum networks with weakly-regular architectures. Such networks are capable of idealising end-to-end performance whilst remaining sufficiently realistic. In this way, we may investigate the effectiveness of large-scale networks with consistent connective properties, and unveil the global conditions under which end-to-end rates remain analytically computable. Furthermore, by comparing the performance of ideal, ground-based quantum networks with satellite quantum communication protocols, we can establish conditions for which satellites can be used to outperform fibre-based quantum infrastructure.
The quantum internet is envisaged to connect quantum systems at a global scale to provide diverse applications unimaginable with the classical internet. However, the technological shortcomings and practical challenges of noisy intermediate-scale quantum (NISQ) devices impose several constraints on the vision of a truly global quantum internet. Recent experimental developments for quantum networks are laying the pathway from small-scale to intermediate-scale noisy quantum networks dedicated to specific operational tasks. Although these networks are not at the global scale, they are capable of providing a clear quantum advantage in their dedicated tasks. We explore the cardinal features of these NISQ networks, discuss their design constraints, and explore the possible applications they provide. We also outline some technical challenges that need to be addressed for the practical implementation of these networks and eventually lead toward a truly global, fully functional quantum internet.
Consensus algorithms are mechanisms driving nodes of a network to agreement. In quantum networks they also redistribute information encoded in the global quantum state, allowing global properties to be inferred from local measurements without reconstructing the global state. Motivated by this, we investigate decentralized quantum consensus in qudit networks governed by discrete random unitary dynamics. We characterize the analytic asymptotic dynamics, identify global information preserved by the evolution, and derive conditions for which different notions of quantum consensus emerge. A general theorem is derived that establishes a unified framework for extracting globally encoded information from local measurements beyond consensus states. First, we analyze CNOT networks and determine which global information can be recovered by individual nodes and which requires joint measurements on cooperating nodes. The CNOT model is well understood and presents a case with entanglement creation - a characteristic quantum feature of networks worth analyzing in detail. Second, we identify the complete class of two-qubit interactions that asymptotically achieve consensus on the average number of excitations, thereby establishing a quantum analogue of classical average consensus. We further show that joint measurements on k cooperating nodes recover exactly the first k moments of the excitation-number distribution.
The development of a future global quantum communication network (or quantum Internet) will enable high-rate private communication and entanglement distribution over very long distances.However, the large-scale performance of ground-based quantum networks (which employ photons as information carriers through optical fibers) is fundamentally limited by fiber quality and link length, with the latter being a primary design factor for practical network architectures.While these fundamental limits are well established for arbitrary network topologies, the question of how to best design global architectures remains open.In this work, we introduce a large-scale quantum network model called weakly regular architectures.Such networks are capable of idealizing network connectivity, provide freedom to capture a broad class of spatial topologies, and remain analytically treatable.This allows us to investigate the effectiveness of large-scale networks with consistent connective properties and unveil critical conditions under which end-to-end rates remain optimal.Furthermore, through a strict performance comparison of ideal groundbased quantum networks with that of realistic satellite quantum communication protocols, we establish conditions for which satellites can be used to outperform fiber-based quantum infrastructure, rigorously proving the efficacy of satellite-based technologies for global quantum communications.