Squashed entanglement and approximate private states

TL;DR

This paper proves that squashed entanglement bounds the key size in approximate private states, simplifying existing proofs and extending results to infinite-dimensional and multipartite quantum systems.

Contribution

It simplifies and extends proofs that squashed entanglement bounds the distillable key, including for infinite-dimensional and multipartite quantum systems.

Findings

Bound on key size in approximate private states using squashed entanglement

Simplified proofs for finite-dimensional systems

Extended results to infinite-dimensional and multipartite systems

Abstract

The squashed entanglement is a fundamental entanglement measure in quantum information theory, finding application as an upper bound on the distillable secret key or distillable entanglement of a quantum state or a quantum channel. This paper simplifies proofs that the squashed entanglement is an upper bound on distillable key for finite-dimensional quantum systems and solidifies such proofs for infinite-dimensional quantum systems. More specifically, this paper establishes that the logarithm of the dimension of the key system (call it $\log_{2}K$) in an $\varepsilon$-approximate private state is bounded from above by the squashed entanglement of that state plus a term that depends only $\varepsilon$ and $\log_{2}K$. Importantly, the extra term does not depend on the dimension of the shield systems of the private state. The result holds for the bipartite squashed entanglement, and an extension of this result is established for two different flavors of the multipartite squashed entanglement.