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A quantum system composed of two or more subsystems can be in an entangledstate, i.e. a state in which the properties of the global system are welldefined but the properties of each subsystem are not. Entanglement is at theheart of quantum physics, both for its conceptual foundations and forapplications in information processing and quantum communication. Remarkably,entanglement can be "swapped": if one prepares two independent entangled pairsA1-A2 and B1-B2, a joint measurement on A1 and B1 (called a "Bell-StateMeasurement", BSM) has the effect of projecting A2 and B2 onto an entangledstate, although these two particles have never interacted or shared any commonpast[1,2]. Experiments using twin photons produced by spontaneous parametricdown-conversion (SPDC) have already demonstrated entanglement swapping[3-6],but here we present its first realization using continuous wave (CW) sources,as originally proposed[2]. The challenge was to achieve sufficiently sharpsynchronization of the photons in the BSM. Using narrow-band filters, thecoherence time of the photons that undergo the BSM is significantly increased,exceeding the temporal resolution of the detectors. Hence pulsed sources can bereplaced by CW sources, which do not require any synchronization[6,7], allowingfor the first time the use of completely autonomous sources. Our experimentexploits recent progress in the time precision of photon detectors, in theefficiency of photon pair production by SPDC with waveguides in nonlinearcrystals[8], and in the stability of narrow-band filters. This approach isindependent of the form of entanglement; we employed time-bin entangledphotons[9] at telecom wavelengths. Our setup is robust against thermal ormechanical fluctuations in optical fibres thanks to cm-long coherence lengths.

Entangling Independent Photons by Time Measurement