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HybridNet Report Summary

Project ID: 307450
Funded under: FP7-IDEAS-ERC
Country: France

Final Report Summary - HYBRIDNET (Hybrid Quantum Networks)

The HybridNet project aimed at demonstrating building blocks for the development of quantum networks enabling the distribution and processing of quantum information.

In this endeavor the project first explored an optical hybrid implementation of these networks, where the continuous- and discrete-variable tools and concepts are combined to overcome some limitations of the schemes taken individually or to provide novel capabilities. Along the course of the project, we demonstrated new capabilities for resource-efficient non-Gaussian state generation and processing. Thanks to large escape-efficiency optical parametric oscillators and high-efficiency superconducting single-photon detectors, we generated free-traveling Fock states and hybrid qubits with unprecedented fidelity and generation rate, and we also investigated a variety of protocols based on this approach. These works included for instance the witnessing of discrete single-photon entanglement up to 80 km using only local homodyne measurements or the protection of superposition states by squeezing in phase space. We also demonstrated for the first time the realization of hybrid entanglement between particle-like and wave-like optical states. Besides its fundamental significance for the exploration of entanglement and its possible instantiations, this resource holds promises for heterogeneous quantum protocols and networks where the two encodings can be combined or interconverted in the form best suited for a particular process or physical system. This resource was exploited to develop novel protocols for hybrid networks, including the demonstration of remote state preparation of arbitrary continuous-variable qubits and the demonstration of EPR steering. This leap in quantum state engineering provides novel resources for optical hybrid architectures and quantum network operations based on heterogeneous systems.

In parallel, we developed a novel ensemble-based light-matter interface as an efficient all-fibered quantum node. We have built an entirely new set-up where cold atoms are trapped in the vicinity of an optical nanofiber. As a first experiment with this system, we demonstrated the realization of slow light and storage, providing therefore a fibered memory. Then, by using an optical lattice in the evanescent field with a period nearly commensurate with the resonant wavelength, we demonstrated an efficient atomic Bragg mirror in this one-dimensional system. The ability to control photon transport in 1D waveguides coupled to spin systems would enable novel quantum network capabilities and the study of many-body effects emerging from long-range interactions. Finally, we demonstrated the heralded storage of collective excitations in this platform, with the subsequent retrieval into the guided mode. The achievements obtained in this project have shown that this fibered platform is very promising for quantum network and repeater implementation and, more generally, contributed to the emerging field of waveguide-QED. Besides these results, in another setting, i.e., a very elongated high-optical-depth cold atom trap, we also proposed and implemented a multiple-degree-of-freedom quantum memory for light that holds promise of increased network capacity and we demonstrated the most efficient memory to date for optical qubits, with a storage-and-retrieval efficiency of 70% and a fidelity above 99%.

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