  A quantum system composed of two or more subsystems can be in an entangled
state, i.e. a state in which the properties of the global system are well
defined but the properties of each subsystem are not. Entanglement is at the
heart of quantum physics, both for its conceptual foundations and for
applications in information processing and quantum communication. Remarkably,
entanglement can be "swapped": if one prepares two independent entangled pairs
A1-A2 and B1-B2, a joint measurement on A1 and B1 (called a "Bell-State
Measurement", BSM) has the effect of projecting A2 and B2 onto an entangled
state, although these two particles have never interacted or shared any common
past[1,2]. Experiments using twin photons produced by spontaneous parametric
down-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 sharp
synchronization of the photons in the BSM. Using narrow-band filters, the
coherence time of the photons that undergo the BSM is significantly increased,
exceeding the temporal resolution of the detectors. Hence pulsed sources can be
replaced by CW sources, which do not require any synchronization[6,7], allowing
for the first time the use of completely autonomous sources. Our experiment
exploits recent progress in the time precision of photon detectors, in the
efficiency of photon pair production by SPDC with waveguides in nonlinear
crystals[8], and in the stability of narrow-band filters. This approach is
independent of the form of entanglement; we employed time-bin entangled
photons[9] at telecom wavelengths. Our setup is robust against thermal or
mechanical fluctuations in optical fibres thanks to cm-long coherence lengths.
