  Broadband quantum memory is critical to enabling the operation of emerging
photonic quantum technology at high speeds. Here we review a central challenge
to achieving broadband quantum memory in atomic ensembles -- what we call the
'linewidth-bandwidth mismatch' problem -- and the relative merits of various
memory protocols and hardware used for accomplishing this task. We also review
the theory underlying atomic ensemble quantum memory and its extensions to
optimizing memory efficiency and characterizing memory sensitivity. Finally, we
examine the state-of-the-art performance of broadband atomic ensemble quantum
memories with respect to three key metrics: efficiency, memory lifetime, and
noise.
