  Quantum memories are an important building block for quantum information
processing. Ideally, these memories preserve the quantum properties of the
input. We present general criteria for measures to evaluate the quality of
quantum memories. Then, we introduce a quality measure based on coherence
satisfying these criteria, which we characterize in detail for the qubit case.
The measure can be estimated from sparse experimental data and may be
generalized to characterize other building blocks, such as quantum gates and
teleportation schemes.
