Owing to a reduced solar background and low propagation losses in the atmosphere, the 2- to 2.5-$\mu \mathrm{m}$ 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 $2.1\mu \mathrm{m}$ 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 $2.1\mu \mathrm{m}$.

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• Optical quantum information processing
• Photon pairs & parametric down-conversion
• Quantum communication
• Quantum cryptography
• Quantum entanglement
• Quantum measurements
• Quantum optics
• Quantum tomography