Correlated Catalysis Removes the Strong Converse for Entanglement Distillation

Abstract

Entanglement distillation converts noisy shared quantum states into maximally entangled pairs using local operations and classical communication. The distillable entanglement $E_D$ determines the maximum asymptotic rate at which this process can be performed reliably. For several classes of quantum states, distillation above this rate obeys a strong converse: the fidelity with the target state vanishes asymptotically. Here we show that correlated catalysis removes the strong converse for every distillable bipartite state. Specifically, at any target rate $R>E_D$, the optimal asymptotic fidelity is given by $E_D/R$. Thus, although catalysis does not increase the maximum reliable distillation rate, it allows nonvanishing fidelity at every finite rate. Remarkably, this advantage persists even when the catalyst is returned exactly at every finite blocklength and the correlations between the output and catalyst vanish asymptotically. We further characterize the catalyst size required to sustain nonvanishing fidelity above the reliable distillation rate.