Quantum frequency conversion for a quantum internet testbed

At Quantnet Testbed, we aim to connect two ion traps located at LBNL and UCB over a fiber link of 10km. These ion traps emit 854 nm photons, with their polarizations entangled with the ion states. To reduce transmission loss, these photons must be converted to telecom wavelengths. Here, we employ Quantum frequency conversion (QFC) to convert these photons from 854 nm to 1550 nm using 1902 nm pump light via difference frequency generation (DFG) and we report a preliminary conversion efficiency of 12%. Further, the converted photons at 1550 nm are sent using fiber links to a Bell state measurement where photons are projected onto a Bell state to entangle the ions. For a successful Bell state measurement, photons must be indistinguishable in all degrees of freedom. We ensure temporal and spatial indistinguishability of photons using Hong-Ou-Mandel interference and single-mode fibers, respectively. Additionally, we achieve polarization indistinguishability by stabilizing the fiber link, maintaining photon polarization with >99.5% fidelity over 24 hours. In summary, we develop fundamental building blocks for connecting not only ion traps but also other quantum nodes within the Quantnet testbed.