Quantum interfaces that generate entanglement or correlations between a photon and an atomic memory are fundamental building blocks in quantum repeater research. Temporal, spatial, and spectral multiplexed atom-photon entanglement interfaces in cold atomic systems based on spontaneous Raman scattering processes, offer an effective technical approach to realizing quantum repeaters. Compared with the other schemes, temporal-multiplexing schemes are particularly attractive since they repeatedly use the same physical process. In these schemes, readout efficiency plays a crucial role. Theoretical models indicate that even a 1% increase in readout efficiency can lead to a 7%-18% improvement in the probability of a long-distance entanglement distribution. However, current temporal-multimode quantum memory implementations often suffer from low readout efficiencies unless optical cavities or large optical-depth atomic ensembles are employed.
In this study, we address this challenge by employing expandable pulsed light fabrication technology and carefully selecting energy level transitions to develop a high-efficiency temporal-multiplexed quantum source. Our approach involves applying a train of write laser pulses to an atomic ensemble from different directions, thereby creating spin-wave memories and Stokes-photon emissions. We designed an expandable pulsed light fabrication device based on the principle of optical path reversibility, allowing a writing laser beam to pass through an acousto-optic modulator (AOM) network in two different directions. This setup enables precise control over the directions of the write pulse train through real-time manipulation of the field-programmable gate array (FPGA) and the diffraction order of the AOMs. In our experiment, we prepared six mode pairs. Upon detection of a Stokes photon during the experimental cycle, the FPGA outputs a feedforward signal after a designated storage time, triggering the application of a corresponding reading pulse from the read AOM network to the atomic ensemble, thereby generating an anti-Stokes photon. To enhance readout efficiency, we optimized the energy level structure of the read pulse transitions, $\left|b \rightarrow e_2\right\rangle$ to $\left|b \rightarrow e_1\right\rangle$; specifically, we adjusted the transition frequencies of the read pulses compared with those used in current temporal-multimode quantum memory schemes. Theoretical calculations showed that when the frequencies of the read pulses are tuned to the transitions $\left|b \rightarrow e_1\right\rangle$ and $\left|b \rightarrow e_2\right\rangle$, the readout efficiencies are about 33% and 15%, suggesting that the chosen energy level transitions could double the readout efficiency.
Experimental results demonstrated a readout efficiency of 38% for the multiplexed source and the Bell parameter of 2.35. Additionally, our setup achieved a 5.83-fold increase in the probability of successful entanglement generation compared to a single channel entanglement source. Our method is cost-effective, easy to operate, and highly applicable. For instance, based on our findings, further improvements in readout efficiency could be realized through cavity-enhanced atom-photon coupling, and entanglement fidelity could be increased by suppressing noise in temporal-multimode memory schemes. This work provides a solid foundation and effective methods for the realization of high-efficiency temporal-multimode quantum memory and the development of large-scale quantum networks.