  Quantum light-matter interfaces are at the heart of photonic quantum
technologies. Quantum memories for photons, where non-classical states of
photons are mapped onto stationary matter states and preserved for subsequent
retrieval, are technical realizations enabled by exquisite control over
interactions between light and matter. The ability of quantum memories to
synchronize probabilistic events makes them a key component in quantum
repeaters and quantum computation based on linear optics. This critical feature
has motivated many groups to dedicate theoretical and experimental research to
develop quantum memory devices. In recent years, exciting new applications, and
more advanced developments of quantum memories, have proliferated. In this
review, we outline some of the emerging applications of quantum memories in
optical signal processing, quantum computation, and nonlinear optics. We review
recent experimental and theoretical developments, and their impacts on more
advanced photonic quantum technologies based on quantum memories.
