  Quantum-enhanced optical systems operating within the 2- to 2.5-$\mu$m
spectral region have the potential to revolutionize emerging applications in
communications, sensing, and metrology. However, to date, sources of entangled
photons have been realized mainly in the near-infrared 700- to 1550-nm spectral
window. Here, using custom-designed lithium niobate crystals for spontaneous
parametric down-conversion and tailored superconducting nanowire single-photon
detectors, we demonstrate two-photon interference and polarization-entangled
photon pairs at 2090 nm. These results open the 2- to 2.5-$\mu$m mid-infrared
window for the development of optical quantum technologies such as quantum key
distribution in next-generation mid-infrared fiber communication systems and
future Earth-to-satellite communications.
