Quantum memories play a pivotal role in enabling long-distance entanglement by linking short-distance connections, overcoming photon transmission losses. Rare-earth ions doped crystals have emerged as promising systems for quantum memory, with integrated solid-state devices showing particular potential. However, prior implementations were limited to optically excited states, which inherently restrict storage time and retrieval flexibility due to the short lifetime of these states.
The breakthrough lies in the implementation of spin-wave storage. This approach encodes photons into spin-wave excitations in ground states, vastly extending storage times to the spin coherence lifetime and enabling on-demand retrieval. Nevertheless, the challenge of separating single-photon signals from noise caused by strong control pulses has hindered progress in integrated structures - until now.
The research team utilized advanced techniques, including direct femtosecond-laser writing, to fabricate a circularly symmetric waveguide in a europium-doped yttrium orthosilicate (Eu:YSO) crystal.
This design enabled polarization-based noise filtering, combined with temporal gates, spectral filtering, and a counter-propagation configuration, to efficiently isolate the single-photon signal. Two protocols - modified noiseless photon echo (NLPE) and atomic frequency comb (AFC) - were implemented for signal retrieval, with NLPE achieving over four times the efficiency of AFC under identical conditions.
The device demonstrated an impressive fidelity of 94.9+/-1.2% when storing and retrieving time-bin qubits encoded with single-photon inputs, surpassing the theoretical limits of classical devices. This accomplishment underscores the reliability of the integrated spin-wave quantum memory.
This advancement lays the groundwork for multiplexed quantum repeaters in integrated formats and high-capacity, transportable quantum memories. The findings were published in the November 2024 issue of National Science Review under the title "Integrated Spin-wave Quantum Memory," with co-corresponding authors Prof. Zong-Quan Zhou and Prof. Chuan-Feng Li, and co-first authors Dr. Tian-Xiang Zhu and graduate student Ming-Xu Su.
Research Report:Integrated spin-wave quantum memory
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