No cardinality bound for squashed entanglement

2026-09-12

AI Summary

本文解决了squashed entanglement优化过程中条件系统E的维度是否有限制的问题，通过直接求和步骤证明了不存在这样的维度限制。

Abstract

Squashed entanglement is an additive bipartite entanglement measure. For a state $ρ_{AB}$, it is defined as the infimum of all conditional mutual information $I(A:B\mid E)$ evaluated on extensions $ρ_{ABE}$. It was unresolved whether there is a bound on the dimension of the conditioning system, $E$, needed for this optimization. We show that no such cardinality bound is possible. Explicitly, we find that the squashed entanglement for a partially dephased Bell pair on two qubits cannot be achieved for any finite dimensional extension $E$. A crucial ingredient of this proof is a direct sum step, which in a sense, symmetrizes two purifying systems while keeping the conditional mutual information unchanged and the coherence no worse.

Problem

• squashed entanglement
• cardinality bound
• conditional mutual information

Innovation

• squashed entanglement
• conditional mutual information
• direct sum step