Extraction of correlated 2-photons with near unity efficiency

We report a source of 2-photons that can be extracted with near unit efficiency. The reduced mode area of solid-core microstructure fibers lets a light pulse induce significant nonlinear optical interaction inside a short fiber, making it easy to generate 2-photon entanglement. However, the photon extraction efficiency is low due to the small core size ($d\sim $1 $\mu $m) that requires high numerical aperture (NA) lenses to couple light in and out of the fiber. Tapering the core at the fiber end to 10 $\mu $m allows the use of anti-reflection-coated lenses of smaller NA, to achieve a single-photon extraction efficiency of \textit{$\eta $}$_{f}$ = 96{\%}. Using a pair of volume holographic gratings for selecting any wavelength of interest increased our spectral transmittance for that wavelength to \textit{$\eta $}$_{g}$ = (98{\%})$^{2}$, enabling a near unit efficiency in extracting a single photon from the fiber source: \textit{$\eta $}$_{f}$\textit{$\eta $}$_{g}$ = 92.2{\%}. The final 2-photon detection efficiency of 10{\%} includes the efficiencies of single-photon detection modules ($\sim $ 70{\%} each) and single-mode fiber collection ($\sim $ 50{\%} per channel). At an average pump power of $P$ = 50 $\mu $W and a laser repetition rate of $R$ = 76 MHz, we detect 50 photon pairs s$^{-1}$ with $g^{(2)}$(0) = 0.0055 and a coincidence-to-accidental ratio of 900:1. Higher pair rates at the same $g^{(2)}$ level can be achieved by increasing $R. $With better photon detection, this source may enable loophole-free Bell tests.