Entanglement swapping over 30 km in a three-node metropolitan quantum network in New York City

Entangling two particles over metropolitan-scale distances is a challenging endeavor, especially through existing infrastructure. Entanglement swapping offers an enticing method to realize this goal. We report on our work on entanglement swapping in a three-node quantum network between lower Manhattan and Brooklyn using our bichromatic rubidium-based entanglement sources, which generate entangled photon pairs at 795 nm (near-infrared) and 1324 nm (telecom O-band) [A. Craddock, et al. Phys. Rev. Applied 21, 034012 (2024)]. 

We can repeat the swapping experiment under different conditions: using superconducting nanowire detectors for all photons in a single location, and then using single-photon avalanche diode (SPAD) detectors for the 795 nm photons both in a single location and with our gear deployed over the metropolitan network with some 16 km of fibers between the intermediate node and each endpoint. 

The use of SPADs makes the implementation of quantum network easier and more cost effective. However, their large jitter (~ 350 ps) represents an experimental hurdle. By using temperature-tunable etalons to shape the output photons of our entanglement source and make them compatible with SPADs, we make important progresses towards the practical implementation of metropolitan-scale quantum networks.