Fig. 2: Experimental setup and photon counting histogram.

a Schematic of the experimental setup. The signal beam and the pump beam for the memory crystal (MC) counter propagate with each other and overlap on the MC with a small angle. The signal beam is attenuated to the single-photon level by neutral density filters (NFDs). The readout signal is collected by a single-mode fiber and passes through two acoustic-optic modulators (AOM) to shift the optical frequency by 400 MHz. The signal is spectrally filtered by the filter crystal (FC) and two 1-nm band-pass filters (BPF). The FC is doubled passed with the help of a polarization beam splitter (PBS) and a Faraday rotator (FR) and the signal is finally detected by a single photon detector (SPD). b Photon counting histogram for storage of weak coherent pulses with an average photon number of 1.17 ± 0.11 photons per pulse. The blue and red lines correspond to measurements with input and without input, respectively. The input (black line) is also included for reference. The input and signal data inside the shadowed area is minified by five times. The bin size is 262 ns, and the experiment is repeated for 50,000 trials.