Figure 4: Characterization of the quantum memory-quantum frequency converter combined setup.

Cross-correlation between Stokes and converted anti-Stokes photons as a function of Stokes detection probability, for a pump power of 140 mW (red plain squares, left axis). The number of trials for this measurement varies from 10⁷ to 10⁸. The shaded area represents the expected . It is deduced from the data of Fig. 2b by means of equation (1) (see Methods). The half width represents one s.d. around the mean value. The calculated points have been interpolated with a spline. The measured Cauchy-Schwarz parameter R^c is plotted on the right axis (blue empty circles). The unconditional autocorrelation measurement for the converted anti-Stokes field has been measured for a pump power of 120 mW. The error bars represent ±1 s.d. assuming Poissonian statistics for the photon counts.