On the structure of LOCC: finite vs. infinite rounds

TL;DR

This paper explores the structure of LOCC measurements, showing that finite and infinite-round measurements are densely interconnected, with finite-round measurements approximable by infinite-round sequences and vice versa, revealing a nuanced relationship.

Contribution

It establishes that the set of LOCC measurements requiring infinite rounds is dense in all LOCC, and introduces a simple necessary condition for finite-round LOCC.

Findings

Finite-round LOCC measurements are limits of infinite-round sequences.

Infinite-round LOCC measurements can be approximated by finite-round sequences.

A new simple necessary condition for finite-round LOCC is proposed.

Abstract

Every measurement that can be implemented by local quantum operations and classical communication (LOCC) using an infinite number of rounds is the limit of a sequence of measurements each of which requires only a finite number of rounds. This rather obvious and well-known fact is nonetheless of interest as it shows that these infinite-round measurements can be approximated arbitrarily closely simply by using more and more rounds of communication. Here we demonstrate the perhaps less obvious result that (at least) for bipartite systems, the reverse relationship also holds. Specifically, we show that every finite-round bipartite LOCC measurement is the limit of a continuous sequence of LOCC measurements, where each measurement in that sequence can be implemented by LOCC, but only with the use of an infinite number of rounds. Thus, the set of LOCC measurements that require an infinite number of rounds is dense in the entirety of LOCC, as is the set of finite-round LOCC measurements. This means there exist measurements that can only be implemented by LOCC by using an infinite number of rounds, but can nonetheless be approximated closely by using one round of communication, and actually in some cases, no communication is needed at all. These results follow from a new necessary condition for finite-round LOCC, which is extremely simple to check, is very easy to prove, and which can be violated by utilizing an infinite number of rounds.