High-brightness fiber-coupled source of polarization-entangled photon pairs spanning the telecom C- and L-bands

We report a compact, flexible, and efficient fiber-integrated broadband source of polarization-entangled photon pairs at telecommunication wavelengths spanning the C- and L-bands. Type-0 spontaneous parametric down-conversion (SPDC) in a 4-cm long fiber-pigtailed Zn-indiffused MgO-doped periodically-poled lithium-niobate (PPLN) ridge waveguide is employed in the Sagnac configuration. The source generates a broadband spectrum centered at 1550-nm with full-width at half-maximum (FWHM) ∼ 52 nm and spectral brightness ∼ 2 × 108 pairs/s/mW/nm. The generated broadband (unfiltered) polarization-entangled state shows two-photon interference in the mutually unbiased bases with minimum fringe visibility > 86% and raw concurrence ∼ 0.83 (∼ 0.88 after background subtraction). The raw fidelity of the generated Bell-state is > 89% (∼92.2% after background subtraction) and it violates the CHSH inequality (S = 2.49 ± 0.017) by ∼ 29 standard deviations. The broadband source offers spectral tunability and can be used in wavelength-division-multiplexing (WDM) based quantum key distribution (QKD) and communication networks.