  In this paper, we study the number of rounds of communication needed to
implement certain tasks by local quantum operations and classical communication
(LOCC). We find that the class of LOCC operations becomes strictly more
powerful as more rounds of classical communication are permitted. Specifically,
for every $n$, there always exists an $n$ round protocol that is impossible to
implement in $n-2$ rounds. Furthermore, we show that certain entanglement
transformations are possible if and only if the protocol uses an infinite
(unbounded) number of rounds. Interestingly, the number of rounds required to
deterministically distill bipartite entanglement from a single multipartite
state can be strongly discontinuous with respect to the amount of entanglement
distilled.
