Figure 2

Performance of the FORD memory and measured enhancement with or without feedback control. (a) The cross-correlation measurement. The depicted data are obtained under the condition of one write pulse and one read pulse. The cross-correlation function \(g_{\mathrm {S}-\mathrm {AS}}^{(2)}\) decreases due to the decoherence arising from the atomic motion. And the data are fitted by the function \(g_{\mathrm {S}-\mathrm {AS}}^{(2)}=1+C/\left( 1+\mathrm {At}^{2}+\mathrm {Bt}\right)\), in which the quadratic term originates from the random atomic motion and the linear term results from the background noise of incident pulses. Error bars are derived from the Poisson distribution of photons. (b) The enhancement of generation rate as a function of the number of write pulses. Here the data in red dots correspond to the results with feedback control, while the data in the blue diamonds are obtained from the experiments without feedback control. The enhancement is defined as \(E={R_{k}}/{R_{1}}\), where the \(R_{1}\), \(R_{k}\) represents the photon generation rate in the experiment with one and k write pulses respectively. (c) The possible coincidence counts between different pulses in two experiments. The data in both cases are obtained with an efficient measurement period of 300 s, which corresponds to \(10^{8}\) experiments preformed. In the horizontal axis labeled by write (read) sequence, the number n represents the nth write pulse, and in the read sequence R means the applied read pulse. The measured counts of anti-Stokes photons, which are converted from the collective excitation generated by the same order write pulse, is connected with a dashed line.