The synchronization function of quantum memories can be employed to connect multiple short-distance entanglement into long-distance entanglement, so that to effectively overcome the transmission loss of photons and enable the construction of large-scale quantum networks. The rare-earth ions doped crystals is a candidate system for implementation of quantum memories with excellent performances, and integrated solid-state quantum memories have been successfully demonstrated with various micro- and nano- fabrication techniques.
All previous demonstrations of integrated quantum memories for light are limited to the storage in the optically-excited states, which does not support on-demand retrieval with continuously adjustable storage times, and the storage time is fundamentally limited by the excited-state lifetime. The spin-wave storage, which stores photons into the spin-wave excitation in ground states, could enable on-demand retrieval with a storage time extended to the spin coherence lifetime. However, separating the single-photon-level signal from the large amount of noise induced by strong control pulses is a formidable challenge in such integrable structures. The spin-wave quantum storage has yet to be demonstrated in integrable solid-state devices and has been considered as a principal obstacle towards practical applications of integrated quantum memories.
Recently, the group led by Chuan-Feng Li and Zong-Quan Zhou at University of Science and Technology of China has successfully demonstrated an integrated spin-wave quantum memory, by implementing spin-wave quantum storage protocols using a specially developed device. To suppress the noise, the group employed direct femtosecond-laser writing to fabricate a circularly-symmetric waveguide in a Eu:YSO crystal, to enable the polarization-based filtering of noise in the integrated device. Combined with filtering techniques in other degrees of freedom, including temporal gates, spectral-filtering crystals and a counter-propagation configuration, the single-photon-level signal can co-propagate with strong control pulses in the same waveguide and can be efficiently separated. To retrieve the signal, the group implemented two spin-wave storage protocols, namely, a modified noiseless photon echo (NLPE) and the full atomic frequency comb (AFC) protocol. Under the same experimental configuration, NLPE provides an efficiency enhancement of more than 4 times as compared to the AFC, due to the well-preserved sample absorption in the NLPE memory. Finally, time-bin qubits encoded with single-photon-level inputs are stored and retrieved with a fidelity of 94.9±1.2%, which is far beyond the maximal fidelity that can be obtained with any classical device, demonstrating the reliability of this integrated device.
This demonstration of the spin-wave integrated quantum memory has been a long-expected goal, and lays out the foundations for the construction of multiplexed quantum repeaters in an integrated configuration and high-capacity transportable quantum memories. These results have been published in National Science Review 2024, Issue 11, 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.
