Long-distance entanglement distribution through satellite intermediary entanglement swapping

Efficient transcontinental entanglement distribution is necessary to build a global quantum network. Without quantum repeaters, distribution through optical fibers is assailed by loss and scattering, limiting the network's reach to around 100 kilometers. This distance can be greatly extended however, by transmitting photons through free space, where the transmission falls only as the reciprocal square of the propagation distance. Our experiment aims to prove the viability of one proposed satellite intermediary scheme: a down-link architecture using entanglement swapping. In this scheme, a satellite generates a pair of entangled photons that are spectrally unentangled - and therefore able to interfere with other photons. The satellite transmits the telecom pump and the entangled telecom photons down to a station on Earth's surface; by doing so, both channels experience the same temporal drift due to Doppler shift and dispersion. The transmitted pump can then be collected, reamplified, and used to pump a second terrestrial entanglement source. Synchronizing and interfering the satellite and terrestrial entangled, telecom photons will then swap entanglement to the unused photons. We are implementing this by pumping non-degenerate entanglement sources, which produce daughter photons at 773 nm and 1588 nm, with a 520 nm pump, generated from third harmonic generation.