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Efficient storage of telecom-band quantum optical information represents a crucial milestone for establishing distributed quantum optical networks. Erbium ions in crystalline hosts provide a promising platform for telecom quantum memories; however, their practical applications have been hindered by demanding operational conditions, such as ultra-high magnetic fields and ultra-low temperatures. In this work, we demonstrate the storage of telecom photonic qubits encoded in polarization, frequency, and time-bin bases. Using the atomic frequency comb protocol in an Er³⁺-doped crystal, we developed a memory initialization scheme that improves storage efficiency by over an order of magnitude. The observed results were made possible by the deliberate selection of the pumping sequence and the minimization of lattice interactions, to the extent possible without the use of dilution refrigerators or superconducting magnets. 

Abstract We investigate the optical lifetime, decay characteristics, spectral linewidth and energy level properties of thulium ions doped in a KY(WO ${}_{\text{4}}$) ${}_{\text{2}}$crystal at 4 K temperature. High doping concentration of thulium ions with inhomogeneous broadening allow us to study nonradiative behaviors, instantaneous spectral diffusion, and spectral power broadening in this solid-state material. The theoretical consideration of ion–ion interactions is shown to accurately characterize the absorption, decay and other spectral behaviors of Tm ${}^{\text{3}+}$ions. We observe more than ten-fold reduction in the decay time of³H₄state and about three-fold reduction in the spectral-hole lifetime as we approach the center of the inhomogeneous broadening, corresponding to higher optical densities. 

Quantum memory devices with high storage efficiency and bandwidth are essential elements for future quantum networks. Solid-state quantum memories can provide broadband storage, but they primarily suffer from low storage efficiency. We use passive optimization and algorithmic optimization techniques to demonstrate nearly a sixfold enhancement in quantum memory efficiency. In this regime, we demonstrate coherent and single-photon-level storage with a high signal-to-noise ratio. The optimization technique presented here can be applied to most solid-state quantum memories to significantly improve the storage efficiency without compromising the memory bandwidth. Published by the American Physical Society2024 

Optical photons are powerful carriers of quantum information, which can be delivered in free space by satellites or in fibers on the ground over long distances. Entanglement of quantum states over long distances can empower quantum computing, quantum communications, and quantum sensing. Quantum optical memories are devices designed to store quantum information in the form of stationary excitations, such as atomic coherence, and are capable of coherently mapping these excitations to flying qubits. Quantum memories can effectively store and manipulate quantum states, making them indispensable elements in future long-distance quantum networks. Over the past two decades, quantum optical memories with high fidelities, high efficiencies, long storage times, and promising multiplexing capabilities have been developed, especially at the single-photon level. In this review, we introduce the working principles of commonly used quantum memory protocols and summarize the recent advances in quantum memory demonstrations. We also offer a vision for future quantum optical memory devices that may enable entanglement distribution over long distances.