Fig. 4: Nuclear spin entanglement distribution through 35 km of deployed fibre.
From: Entanglement of nanophotonic quantum memory nodes in a telecom network
a, Schematic of QFC setup. At node A, the photonic qubit is downconverted from 737 nm to 1,350 nm, which can propagate with low loss in telecom single-mode fibres. At the node B, it is upconverted back to 737 nm. The pump laser frequencies in the upconversion and downconversion setups are detuned by Δω = 13 GHz to compensate for the difference in optical frequencies of the two SiVs. b, Nuclear spin Bell-state fidelities for varying lengths of telecom fibre spools between the two nodes. Entanglement persists for fibre lengths up to 40 km. Bell-state decoherence can be explained by a model incorporating a decrease in signal-to-noise ratio because of dark counts at 2.7 Hz and conversion noise photons at 2.5 Hz (solid line). The dashed line shows the classical limit. c, Measurement results of Bell-state measurement of \({\left|{\varPhi }_{{\rm{nn}}}^{-}\right\rangle }^{{\rm{ED}}}\) state created through a 35-km long deployed fibre link shown in d, resulting in a fidelity of \({{\mathcal{F}}}_{\left|{\varPhi }_{{\rm{nn}}}^{-}\right\rangle }^{{\rm{ED}}}=0.69(7)\). Dashed bars show correlations predicted by a theoretical model using independently measured performance parameters of our system. d, Route of the deployed fibre link connecting nodes A and B. It consists of 35 km deployed telecom fibre routed towards and back from an off-site location, crossing four municipalities in the greater Boston metropolitan region. Error bars in b and c are 1 s.d. Scale bar, 1,000 m (d).
