We explain how to share photons between two distant parties using concatenated entanglement swapping and assess performance according to the two-photon visibility as the figure of merit. From this analysis, we readily see the key generation rate and the quantum bit error rate as figures of merit for this scheme applied to quantum key distribution (QKD). Our model accounts for practical limitations, including higher-order photon pair events, dark counts, detector inefficiency, and photon losses. Our analysis shows that compromises are needed among the runtimes for the experiment, the rate of producing photon pairs, and the choice of detector efficiency. From our quantitative results, we observe that concatenated entanglement swapping enables secure QKD over long distances but at key generation rates that are far too low to be useful for large separations. We find that the key generation rates are close to both the Takeoka–Guha–Wilde and the Pirandola–Laurenza–Ottaviani–Banchi bounds.
We present our approach for sharing photons and assessing resultant four-photon visibility between two distant parties using concatenated entanglement swapping. In addition we determine the corresponding key generation rate and the quantum bit-error rate. Our model is based on practical limitations of resources, including multipair parametric down-conversion sources, inefficient detectors with dark counts and lossy channels. Through this approach, we have found that a trade-off is needed between experimental run-time, pair-production rate and detector efficiency. Concatenated entanglement swapping enables huge distances for quantum key-distribution but at the expense of low key generation rate.