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Letter

Nature Physics 3, 692–695 (1 October 2007) | doi:10.1038/nphys700

Entangling independent photons by time|[nbsp]|measurement

Matth|[auml]|us Halder , Alexios Beveratos , Nicolas Gisin , Valerio Scarani , Christoph Simon & Hugo Zbinden

Entanglement is at the heart of quantum physics, both for its conceptual foundations and for applications in quantum communication. Remarkably, entanglement can be |[lsquo]|swapped|[rsquo]|: if we prepare two independent entangled pairs A1–A2 and B1–B2, a joint measurement on A1 and B1 (called a |[lsquo]|Bell-state measurement|[rsquo]|, BSM) has the effect of projecting A2 and B2 onto an entangled state, although these two particles have never interacted nor share any common past.