Entanglement Swapping at Fermilab

Entanglement swapping is a key protocol for quantum communication technologies such as quantum repeaters and quantum key distribution. We perform entanglement swapping at Fermilab National Accelerator Laboratory using state-of-the-art superconducting nanowire single photon detectors (SNSPDs) and off-the-shelf fiber-coupled components. Bell pairs of time-bin qubits are generated by pumping periodically-poled lithium niobate crystals with pulses of 1536 nm light at a repetition rate of 100 MHz. We first measure the Hong-Ou-Mandel visibility of the interfering photons, which corresponds to a photon indistinguishability of 0.9. Next, we measure the entanglement swapping fidelities in the X(Z) basis with(out) interferometers added to the heralded photon paths. We support our results with a theoretical model derived using phase space methods, which encompasses all experimental imperfections, including loss, multiphoton effects, and photon distinguishability. Our system is a step towards the US Department of Energy’s proposed quantum internet backbone connecting the Office of Science National Labs.