Local Transformations Requiring Infinite Rounds of Classical Communication

Abstract: 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.