Long-distance entanglement swapping with photons from separated sources

We report the experimental realization of entanglement swapping over long distances in optical fibers. Two photons separated by more than $2\mathrm{km}$ of optical fibers are entangled, although they never directly interacted. We use two pairs of time-bin entangled qubits created in spatially separated sources and carried by photons at telecommunication wavelengths. A partial Bell-state measurement is performed with one photon from each pair, which projects the two remaining photons, formerly independent onto an entangled state. A visibility high enough to infer a violation of a Bell inequality is reported, after both photons have each traveled through $1.1\mathrm{km}$ of optical fiber.

Figure 1

Scheme of principle of entanglement swapping.

Figure 2

Experimental setup. The pump laser is a mode-locked femtosecond Ti-sapphire laser producing $200\mathrm{fs}$ pulses at a wavelength of $710\mathrm{nm}$ with a repetition rate of $75\mathrm{MHz}$. After the crystals, the pump beams are blocked with silicon filters (SF). The Faraday mirrors (FM) are used to compensate polarization fluctuations in the fiber interferometers.

Figure 3

Two-photon interferences for swapped photons, as a function of the phase of one interferometer. The plain squares represent the detection between photons $A$ and $D$, without conditioning on the BSM. The errors bars are too small to be represented. The open circles represent four-photon coincidences, i.e., two-photon interference conditioned on a BSM.