  An optical quantum memory can be broadly defined as a system capable of
storing a useful quantum state through interaction with light at optical
frequencies. During the last decade, intense research was devoted to their
development, mostly with the aim of fulfilling the requirements of their first
two applications, namely quantum repeaters and linear-optical quantum
computation. A better understanding of those requirements then motivated
several different experimental approaches. Along the way, other exciting
applications emerged, such as as quantum metrology, single-photon detection,
tests of the foundations of quantum physics, device-independent quantum
information processing and nonlinear processing of quantum information. Here we
review several prospective applications of optical quantum memories with a
focus on recent experimental achievements pertaining to these applications.
This review highlights that optical quantum memories have become essential for
the development of optical quantum information processing.
