Fig. 3: Memory efficiency measurement.

Two-photon coincidence detection histograms for an AFC storage experiment with 1/Δ = 5 μs (red solid line), and for a reference measurement through a transparency window burned into the memory crystal (blue dashed line). The shaded areas (see insets) were used to calculate the efficiency, where the echo (reference) integration window was 20 ns (15 ns). Total integration times were 1.15 min (echo) and 0.6 min (reference), respectively.