Locally undetermined states, generalized Schmidt decomposition, and an application in distributed computing

TL;DR

This paper characterizes pure multipartite quantum states that cannot be uniquely identified by their reduced states, relates this to a generalized Schmidt decomposition, and applies these findings to fault-tolerant distributed consensus protocols.

Contribution

It provides necessary and sufficient conditions for local undeterminability and links this to a generalized Schmidt decomposition, with applications in distributed computing.

Findings

Characterization of locally undetermined states

Connection between local undeterminability and generalized Schmidt decomposition

Locally undetermined states enable fault-tolerant consensus protocols

Abstract

Multipartite quantum states that cannot be uniquely determined by their reduced states of all proper subsets of the parties exhibit some inherit `high-order' correlation. This paper elaborates this issue by giving necessary and sufficient conditions for a pure multipartite state to be locally undetermined, and moreover, characterizing precisely all the pure states sharing the same set of reduced states with it. Interestingly, local determinability of pure states is closely related to a generalized notion of Schmidt decomposition. Furthermore, we find that locally undetermined states have some applications to the well-known consensus problem in distributed computation. To be specific, given some physically separated agents, when communication between them, either classical or quantum, is unreliable and they are not allowed to use local ancillary quantum systems, then there exists a totally correct and completely fault-tolerant protocol for them to reach a consensus if and only if they share a priori a locally undetermined quantum state.