  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 machine learning techniques to
demonstrate nearly a 6-fold 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.
