Quantum communication networks: design and applications

Quantum communication is among the key applications of an emerging quantum technology. Extending the range of quantum communication to intercontinental distances is an important and necessary goal to establish a world-wide quantum communication network. Such a quantum internet has widespread potential applications, including secure classical communication, distributed quantum computation or distributed sensing. It can be the backbone of public and scientific applications of quantum mechanical principles. Previous research has concentrated on point-to-point communication, where theoretical methods for long-distance communication based on quantum error correction or distribution and purification of entanglement so-called quantum repeaters- have been developed. These methods allow one in principle to overcome loss and channel noise, and also to deal with local noise and imperfections in the apparatus. Short-distance key distribution as well as building blocks for these approaches, including single photon sources, quantum interfaces and quantum memories have already been experimentally demonstrated. However a full- scale long-range communication network has not yet been implemented. Many important aspects of multi-user quantum communication however remain largely unexplored so far. In particular, a multiparty problem calls for a multi-party solution, where advantages over bipartite approaches can be expected. In this project, we will investigate the design and potential applications of such a long-distance quantum communication network. In particular the project is concerned with the following topics. Multi-party communication networks: We consider the design and optimization of multi- party communication networks based on the direct distribution of different multi-party entangled states. Error tolerance, efficiency and stability of different schemes for regular networks, networks with topological defects and real-world networks will be investigated, and compared to bipartite or point-to-point approaches. Big quantum data communication: Design of novel long-distance communication schemes for multipartite communication of large amounts of quantum data, i.e.scalable schemes with ultra-high communication rates based on combinations of error correction and entanglement purification (quantum repeaters). Quantum network elements: We consider the functionality of quantum analogues of the central elements of classical communication networks, including quantum hub, quantum switch and quantum router. Novel quantum aspects and potentials, as well as different realizations using entangled states or hardware will be developed. Applications: We will investigate different applications for multiparty communications networks, including security aspects for different classical multiparty tasks, applications for distributed quantum computation and distributed sensing networks for metrology.

Quantum networks are based on the usage of quantum effects, and represent the next generation of a world-wide internet promising new and unexplored applications. To realize such quantum networks, it is not sufficient to extend existing technologies - but new and fundamentally different approaches are necessary. Among them are the development of new protocols and novel approaches to network design, where quantum effects and devices are not just integrated in existing networks, but serve as a new design principle. A key problem is concerned with the influence of noise and imperfections - and the development of methods to reduce or overcome resulting limitations. Our investigations are concerned with different aspects of quantum networks, their realization, design and applications, thereby paving the way for their experimental realization. The key results are as follows: Network protocols We have developed several new protocols to reduce the effect of noise, and to certify entangled states. By operating on a larger system of multiple copies, the efficiency is significantly enhanced. We have shown the elementary quantum communication protocls such as teleportation or the quantum repeater can be (re)discovered and optimized by machine learning techniques. Genuine and entanglement-based quantum networks We have introduced entanglement-based quantum networks and shown that such a top-down approach offers new design principles and practical advantages. Networks can be optimized and adapted to the desired functionality. The development of a stack model offers a clear, structured approach to different aspects of quantum networks. We have made quantum networks genuine quantum b introducing a quantum control plane, where classical tasks such as sending or addressing can be performed in coherent quantum superposition. This results in new, unexplored possibilities, and a reduced influence of noise. Applications of quantum networks: quantum metrology & privacy We have investigated distributed quantum sensor networks and developed new methods to deal with spatially correlated noise effects. This allows one to maintain the quantum advantage in high-precision measurements even in the presence of noise and imperfections. Furthermore, we have introduced methods to establish private entanglement.