Experimental study of efficient temporal-multimode Duan-Lukin-Cirac-Zoller storage scheme
Authors: Wen Ya-Fei , Zhuang Yuan-Yuan , Wang Zhi-Qiang , Gao Shi-Hui ,   Department of Physics, Taiyuan Normal University, Jinzhong 030619, China,   College of Computer Science and Technology, Taiyuan Normal University, Jinzhong 030619, China,   Institute of Computational and Applied Physics, Taiyuan Normal University, Jinzhong 030619, China
Published: August 2024

<sec>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, present 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 if the readout efficiency is increased by 1%, the probability of long-distance entanglement distribution will be increased by 7%–18%. However, current implementation of temporal-multimode quantum memory often suffers low readout efficiency unless an optical cavity or an atomic ensemble with a large optical-depth is adopted.</sec><sec>In this study, we solve this challenge by using the expandable pulsed light fabricating technology and carefully selecting energy level transitions, so as to develop an efficient 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 design 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 prepare six pairs of modes. After detecting Stokes photons during the experimental cycle, the FPGA outputs a feedforward signal after a specified 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 optimize the energy level structure of the read pulse transitions, <inline-formula><tex-math id="M1">\begin{document}$ \left| {{{b}} \to {{{e}}_2}} \right\rangle $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240799_M1.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240799_M1.png"/></alternatives></inline-formula> to <inline-formula><tex-math id="M2">\begin{document}$ \left| {{{b}} \to {{{e}}_1}} \right\rangle $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240799_M2.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240799_M2.png"/></alternatives></inline-formula>; specifically, we adjust the transition frequencies of the read pulses by comparing with those used in current temporal-multimode quantum memory schemes. Theoretical calculations show that when the frequencies of the read pulses are tuned to the transitions <inline-formula><tex-math id="M3">\begin{document}$ \left| {{{b}} \to {{{e}}_1}} \right\rangle $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240799_M3.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240799_M3.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M4">\begin{document}$ \left| {{{b}} \to {{{e}}_2}} \right\rangle $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240799_M4.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240799_M4.png"/></alternatives></inline-formula>, the readout efficiencies are about 33% and 15%, suggesting that the chosen energy level transitions can double the readout efficiency.</sec><sec>Experimental results indicate a readout efficiency of 38% for the multiplexed source and the Bell parameter of 2.35. Additionally, our device has a 5.83-fold higher probability of successfully generating entanglement than a single channel entanglement source. Our method is cost-effective, easy to operate, and highly applicable. For instance, based on our findings, the readout efficiency can be further improved through cavity-enhanced atom–photon coupling, and entanglement fidelity can be increased by suppressing noise in temporal-multimode memory schemes. This work provides a solid foundation and effective methods for realizing the high-efficiency temporal-multimode quantum memory and developing the large-scale quantum networks.</sec>