Near-Maximal Two-Photon Entanglement for Optical Quantum Communication at 2.1 μm

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 near-maximally entangled photon pairs at 2.1 mu m that could support device-independent quantum key distribution (DIQKD), assuming sufficiently high channel efficiencies. The state corresponds to a positive secure-key rate (0.254 bits/pair, with a quantum bit error rate of 3.8%) based on measurements in a laboratory setting with minimal channel loss and transmission distance. This is promising for the future implementation of DIQKD at 2.1 mu m.