"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.
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."
