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.