  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 can effectively store and manipulate quantum states, which makes them
indispensable elements in future long-distance quantum networks. Over the past
two decades, quantum optical memories with high fidelity, 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.
