Entanglement swapping with independent sources over an optical-fiber network

Abstract

Teleportation of an entangled state, known as entanglement swapping, plays an essential role in quantum communication and network. Here we report a field-test entanglement swapping experiment with two independent telecommunication band entangled photon-pair sources over the optical fibre network of Hefei city. The two sources are located at two nodes 12 km apart and the Bell-state measurement is performed in a third location which is connected to the two source nodes with 14.7 km and 10.6 km optical fibres. An average visibility of 79.9 ± 4.8% is observed in our experiment, which is high enough to infer a violation of Bell inequality. With the entanglement swapping setup, we demonstrate a source independent quantum key distribution, which is also immune to any attack against detection in the measurement site.With the help of quantum entanglement and Bell state measurement (BSM), entanglement swapping [1, 2] can entangle two remote particles sharing no common history without interacting them. Therefore, except for its interest in the fundamental study of quantum information science, entanglement swapping also constitutes a core part for quantum repeater [3,4] in long distance quantum communication [5] and quantum internet [6] for distributed quantum computation [7].For all these quantum communication applications, photons are the most suitable carriers due to its flying nature and robustness against environmental decoherence. In the past two decades, various photonic entanglement swapping experiments have been implemented [8][9][10][11][12]. Recently, a field test of entanglement swapping was demonstrated through free space link [13]. However, in most of the previous demonstrations of entanglement swapping, the same femtosecond laser was used in producing two entangled photon-pair sources [8,9], making them impractical to be implemented in quantum network where the sources should be placed in distant nodes. Pioneering works have been done to synchronize two sources which are pumped independently in laboratory [10][11][12]14]. Owing to the difficulties in guaranteeing the indistinguishability of photons after they are transmitted through the realistic channel, a field test of entanglement swapping with independent entangled photon-pair sources remains an experimental challenge.Besides its applications in quantum repeater, the nonlocal correlation created by entanglement swapping can also be employed in quantum cryptography. Although quantum key distribution (QKD) can in principle provide information-theoretical security based on quantum mechanics, practical QKD systems are suffering from side-channel attacks that explore the device flaws in real-life implementation. Recently, the detector side-channel attacks have been removed by employing measurement-deviceindependent (MDI) QKD protocol [15]. However, potential loopholes still exist on the source side. When the photon source is not well prepared according to the security proof model, attacks would be possible [16].When entanglement swapping is used for QKD, each ...

Limits for quantum networks with semiconductor entangled photon sources

“…The result is shown by the violet curve (5), exhibiting fidelities between 0.9 and 0.99. Curve (6) shows the comparison to the expected probability density if the parameters would not be statistically distributed at all. Therefore the statistical effect of QD properties on entanglement swapping is clear by comparing curves (5) and (6).…”

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