  The first step in quantum information theory is the identification of
entanglement as a valuable resource. The next step is learning how to exploit
this resource efficiently. We learn how to exploit entanglement efficiently by
applying analogues of thermodynamical concepts. These concepts include
reversibility, entropy, and the distinction between intensive and extensive
quantities. We discuss some of these analogues and show how they lead to a
measure of entanglement for pure states. We also ask whether these analogues
are more than analogues, and note that, l o c a l l y, entropy of entanglement
is thermodynamical entropy.
