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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - HELIOS (Hybrid Entanglement of Light for Interconnected Optical Networks)

Teaser

Summary

The project HELIOS realized at Laboratoire Kastler Brossel in Paris focussed on the field of hybrid quantum optics, based on the unique experimental endeavours at the host institution where one of the first resources of hybrid entanglement of light was originally developed. The overall ambition of the project has been to introduce these novel hybrid technologies into the broader, developing framework of quantum networks.
By combining the wave- and particle-like characters of light into a joint entity spread across separate nodes, it has been possible to envision fundamental quantum communication protocols between remote parties operating on different quantum logic â€“ namely, continuous and discrete variable encodings. Two major examples were demonstrated during the project. The first is a quantum steering experiment for one-sided device independent quantum communication between two parties, which has applications in various protocols such as quantum key distribution, randomness generation and entanglement verification. The second is an entanglement swapping experiment required for establishing hybrid links between remote nodes, an important step towards quantum repeaters in heterogeneous networks.

Work performed

\"Quantum steering based on hybrid entanglement.
A first objective of the project has been to verify the fitness of hybrid entanglement of light for quantum steering. The general idea is that, when two parties share a quantum entangled state, measurement on one side can steer the state on the other side in a way that is unpredictable by classical theories. In one-sided device independent scenarios, one untrusted party performs a series of local measurements and publicly announces the outcomes to another party, who has interest in verifying whether they share quantum correlations.
In our experiment we chose the particle-like side of our hybrid entangled state as the untrusted party. The state was subject to local homodyne measurements at different quadrature phases, and the results separated between positive or negative outcomes. At the same time, the wave-like part of the entanglement was also measured by homodyne detection for tomographic reconstruction. Without information from the untrusted party, the state reconstructed on the trusted side would be a mixture. By imposing phase-specific sign-binning from the measurement on the particle-like side, however, the wave-like state could be observed to be physically steered through phase space depending on the measurement chosen by the untrusted party. This qualitative verification of a \"\"spooky-action at a distance\"\" was certified by the violation of a steering inequality. Where classical theories based on local hidden variable would expect the observed correlations to always satisfy a certain inequality, quantum theory allows instead a violation of such inequality. Our observations indicated that the optical hybrid entanglement resource at the host institution could violate this inequality by more than five standard deviation.
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Distribution of hybrid entanglement based on entanglement swapping.
Unlike classical networks, where information can be amplified and regenerated indefinitely, quantum networks depend on teleportation-based protocols to propagate a quantum signal across remote nodes disrupted by lossy channels. A significant part of the project has been devoted to the implementation of an entanglement swapping protocol aimed at distributing hybrid entanglement over two modes that never directly interacted.
A challenge for this experiment involved the creation of a secondary type of entanglement acting as a separate communication channel and interacting with the existing hybrid state. We implemented time-bin multiplexing of the extant resources to create a discrete-variable form of entanglement in concurrence with the original hybrid entanglement of light, and deployed a delay line that would allow the interaction of these two states in preparation for the swapping protocol. The swapping was performed by a Bell-state measurement station based on single-photon subtraction and homodyne conditioning, applied to the one half of the discrete-variable entanglement and the discrete-variable component of the hybrid entanglement. This experiment, which due to the requirement of three coincidental heralding events relied intensely on the high efficiencies available at the host institution for both state generation and measurements, demonstrated the emergence of quantum correlations between the two output modes proved by the presence of non-vanishing negativity of entanglement. The result is a hybrid state, where one half of the purely discrete-variable state originally created is eventually linked to a continuous-variable node, in a successful conversion of the entanglement.\"

Final results

Thanks to the fellowship, Giovanni Guccione pioneered the use of hybrid entanglement of light in the context of quantum networks and quantum communication. It is expected that this novel resource will play a significant role in the development of heterogeneous networks, where operating conditions would dictate the encoding of information in continuous-variable spaces or in discrete-variable qubits depending on the application.