Fiber-based distribution of triggered, entangled, single-photon pairs is a key requirement for the future development of terrestrial quantum networks. In this context, semiconductor quantum dots (QDs) are promising candidates for deterministic sources of on-demand polarization-entangled photon pairs. So far, the best QD polarization-entangled-pair sources emit in the near-infrared wavelength regime, where the transmission distance in deployed fibers is limited. Here, to be compatible with existing fiber network infrastructures, bi-directional polarization-conserving quantum frequency conversion (QFC) is employed to convert the QD emission from 780 nm to telecom wavelengths. We show the preservation of polarization entanglement after QFC (fidelity to Bell state 𝐹𝜙+,𝑐𝑜𝑛𝑣=0.972±0.003) of the biexciton transition. As a step toward real-world applicability, high entanglement fidelities (𝐹𝜙+,𝑙𝑜𝑜𝑝=0.945±0.005) after the propagation of one photon of the entangled pair along a 35.8 km field-installed standard single mode fiber link are reported. Furthermore, we successfully demonstrate a second polarization-conserving QFC step back to 780 nm preserving entanglement (𝐹𝜙+,𝑏𝑎𝑐𝑘=0.903±0.005). This further prepares the way for interfacing quantum light to various quantum memories.
Publication: High-fidelity distribution of triggered polarization-entangled telecom photons via a 36 km intra-city fiber network.
Tim Strobel, Stefan Kazmaier, Tobias Bauer, Marlon Schäfer, Ankita Choudhary, Nand Lal Sharma, Raphael Joos, Cornelius Nawrath, Jonas H. Weber, Weijie Nie, Ghata Bhayani, Lukas Wagner, André Bisquerra, Marc Geitz, Ralf-Peter Braun, Caspar Hopfmann, Simone L. Portalupi, Christoph Becher, and Peter Michler 
Optica Quantum, Vol.2, Article 4
Contact person: T.Strobel
