Optimal recovery for quantum error correction

The calculation of the error threshold of quantum error correcting codes typically proceeds as follows. First, syndromes are measured. Then, a decoder infers the error chain and the corresponding correction is applied. The threshold is then defined as the largest correctable error rate, with the maximum-likelihood decoder corresponding to the "optimal" threshold. However, a broader set of operations could be used to recover quantum information. The true optimal threshold should be optimised over all possible recovery schemes, which can be described by quantum channels. Here, we study such optimal recovery channels and their thresholds p_th^opt. We introduce an information-theoretic quantity, mutual trace distance, which provides a necessary and sufficient diagnostic for sharply determining p_th^opt without explicit optimisation. In contrast, previous works give a lower bound on p_th^opt by specifying particular recovery schemes, e.g. Schumacher-Westmoreland (SW) which provides coherent information as a diagnostic to lower bound p^opt_th. We prove that the Petz and SW recovery schemes are optimal, i.e. their threshold is p_th^opt. With their optimality established, we explore the structure of optimal and non-optimal recovery schemes and their phase diagrams.