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Experimental generation of entangled states for multiparty QKD

Conference key agreement is a multiparty communication scheme where n users try to obtain a common random bit string (e.g., the key) that is guaranteed to be secure from an eavesdropper. One approach is to iterate two-party quantum key distribution protocols among all users followed by classical co-ordination of a shared key (2QKD). However, by using multipartite entanglement the shared key can be distributed to n-users directly (NQKD). Here we implement a proof-of-principle experiment demonstrating salient features of a discrete-variable NQKD protocol in the presence of loss and environmental effects inherent in the use of optical fibre links.

Our approach leverages the design of high-brightness single photon sources in the telecom regime (1550 nm), using spontaneous parametric down-conversion (SPDC) mediated by a nonlinear crystal (ppKTP). We can generate Bell states directly from the source by embedding the ppKTP crystal into a compact Sagnac interferometer. We use two such sources to deliver high generation rates of the four-photon GHZ state, based non-deterministic linear optics approaches, used in the NQKD scheme.