Dispersion-engineered optical fibers for mid-infrared entangled single-photon generation.

Near-infrared (NIR) single-photon sources have become the backbone of quantum technology. There is further potential to demonstrate the next-generation quantum technology applications by extending the wavelengths to the mid-infrared (mid-IR) region. This interest is driven by the extraordinary characteristics of the mid-IR wavelength spectrum. In this paper, we propose germania-doped and ZBLAN fibers that exploit spontaneous four-wave mixing (SpFWM) to generate a single photon-pair in the ∼2 µm to ∼3.5 µm wavelength range. The phase-matching conditions at different wavelengths can be achieved by employing both the same-mode and mixed-mode schemes (intra and intermodal) for the idler, pump, and signal wavelengths. For the intramodal scheme, we exploited the fiber's high NA to produce closely spaced non-degenerate modes (such as LP/LP and LP/LP) and then used combinations of these modes (from LP to LP or LP to LP) for all three signal, pump, and idler wavelengths to achieve the phase-matching. For the intermodal scheme, we borrowed different modes for the signal, pump (including different modes for both pump photons), and idler wavelengths. The mixed-mode schemes allow for a larger core and provide greater flexibility in tuning the idler and signal wavelengths to mitigate the effects of Raman scattering and residual pump photons (∼20-25 THz from the pump frequency), albeit with increased complexity and reduced efficiency. We developed an understanding of the mixed-mode phase-matching scheme and built a model to optimize mode selection for phase-matching. Our investigations reveal, for the first time, the untapped potential of fiber-based mid-IR entangled single photon-pair generation.