Near-maximal two-photon entanglement for quantum communications at 2.1 $mu$m

Owing to a reduced solar background and low propagation losses in the atmosphere, the 2- to 2.5-$mu$m waveband is a promising candidate for daylight quantum communication. This spectral region also offers low losses and low dispersion in hollow-core fibers and in silicon waveguides. We demonstrate for the first time the capability for entanglement-based quantum key distribution (QKD) at 2.1 $mu$m, obtaining a positive secure-key rate (0.417 bits/pair, with a quantum bit error rate of 5.43%) using near-maximally entangled photon pairs in a proof-of-principle device-independent QKD scenario.