High-fidelity entanglement swapping and generation of three-qubit GHZ   state using asynchronous telecom photon pair sources
  We experimentally demonstrate a high-fidelity entanglement swapping and a generation of the Greenberger-Horne-Zeilinger~(GHZ) state using polarization-entangled photon pairs at telecommunication wavelength produced by spontaneous parametric down conversion with continuous-wave pump light. While spatially separated sources asynchronously emit photon pairs, the time-resolved photon detection guarantees the temporal indistinguishability of photons without active timing synchronizations of pump lasers and/or adjustment of optical paths. In the experiment, photons are sufficiently narrowed by fiber-based Bragg gratings with the central wavelengths of 1541~nm and 1580~nm, and detected by superconducting nanowire single-photon detectors with low timing jitters. Observed fidelities are 0.84 \pm 0.04 and 0.70 \pm 0.05 for the entanglement swapping and generation of the GHZ state, respectively. 
Yoshiaki Tsujimoto, Motoki Tanaka, Nobuo Iwasaki, Rikizo Ikuta,   Shigehito Miki, Taro Yamashita, Hirotaka Terai, Takashi Ya...
quant-ph