Generation and detection of polarization entanglement at 2.1 micron

Quantum-enhanced optical systems operating within the 2-2.5 µm spectral region have the potential to revolutionize emerging applications in communications, sensing and metrology. However, until now, sources of entangled photons have been realized mainly in the near-infrared 700-1550 nm spectral window. Above 2 µm lies an atmospheric transparency window with nearly one-third of the solar blackbody radiation as what is typical at telecom wavelengths. There is, therefore, a growing interest to deliver sources and detectors operating in this wavelength range for free-space optical communications and in emerging fields of high-sensitivity metrology, specifically, gravitational wave detection [1,2]. Here, using a 130 fs pulsed laser at wavelength 1.05 μm to pump custom-designed non-linear crystals for spontaneous parametric down-conversion (SPDC) process and tailored superconducting nanowire single-photon detectors (SNSPD), we demonstrate for the first-time two-photon quantum interference and polarization entanglement in the mid-infrared, at ≃ 2.1 μm.