Strong Converse Bounds for Compression of Mixed States
Zahra Baghali Khanian
TL;DR
This work investigates strong-converse bounds for compressing mixed-state quantum sources under visible and blind scenarios. It proves additivity of the entanglement of purification for partially-exchangeable states and derives a pretty strong converse, showing that compression below the optimal rate incurs a sharp fidelity drop. For visible compression of general states, it introduces the $\alpha$-Rényi entanglement of purification $E_{\alpha,p}$ and its regularization to bound fidelity via $\lim_{\alpha\to1^+}E_{\alpha,p}^{\infty}$, while highlighting an open continuity question at $\alpha=1$ that would certify a full strong converse. In the blind setting, it establishes a strong-converse bound assuming a super-unital decoder, which also covers ensembles as a special case. Collectively, the results connect entanglement measures with asymptotic compression limits, advancing understanding of mixed-state source compression.
Abstract
In this paper, we study strong converse properties for both visible and blind compression of mixed states. The optimal rate of a visible compression scheme is obtained in terms of the entanglement of purification, whose additivity remains unknown so far. For a variation of extendible states, we prove that the entanglement of purification is additive and apply this to obtain a "pretty strong" converse bound for the blind and visible compression of such states. Namely, when the rate decreases below the optimal rate, the error exhibits a discontinuous jump from 0 to at least $\frac{1}{3\sqrt{2}}$. To deal with the visible case for general states, we define a new quantity $E_{α,p}(A:R)_ρ$ for a bipartite state $ρ^{AR}$ and $α\in (0,1)\cup (1,\infty)$ as the $α$-Rényi generalization of the entanglement of purification $E_{p}(A:R)_ρ$. For $α=1$, we define $E_{1,p}(A:R)_ρ:=E_{p}(A:R)_ρ$. We show that for any rate below the regularization $\lim_{α\to 1^+}E_{α,p}^{\infty}(A:R)_ρ:=\lim_{α\to 1^+} \lim_{n \to \infty} \frac{E_{α,p}(A^n:R^n)_{ρ^{\otimes n}}}{n}$ the fidelity for the visible compression exponentially converges to zero. Moreover, we consider blind compression of a general mixed-state source $ρ^{AR}$ shared between an encoder and an inaccessible reference system $R$. We obtain a strong converse bound for the compression of this source by assuming that the decoder is a super-unital channel. This immediately implies a strong converse for the blind compression of ensembles of mixed states, by assuming a super-unital decoder, as this is a special case of the general mixed-state source $ρ^{AR}$ where the reference system $R$ has a classical structure.