  In quantum information theory, it is widely believed that entanglement
concentration for bipartite pure states is asymptotically reversible. In order
to examine this, we give a precise formulation of the problem, and show a
trade-off relation between performance and reversibility, which implies the
irreversibility of entanglement concentration. Then, we regard entanglement
concentration as entangled state compression in an entanglement storage with
lower dimension. Because of the irreversibility of entanglement concentration,
an initial state can not be completely recovered after the compression process
and a loss inevitably arises in the process. We numerically calculate this loss
and also derive for it a highly accurate analytical approximation.
