Asymptotic quantification of entanglement with a single copy

Despite the central importance of quantum entanglement in fueling many quantum technologies, the understanding of the optimal ways to exploit it is still beyond our reach, and even measuring entanglement in an operationally meaningful way is prohibitively difficult. This is due to the need to precisely characterise many-copy, asymptotic protocols for entanglement processing. Here we overcome these issues by introducing a new way of benchmarking the fundamental protocol of entanglement distillation (purification), where instead of measuring its asymptotic yield, we focus on the best achievable error. We connect this formulation of the task with an information-theoretic problem in composite quantum hypothesis testing known as generalised Sanov's theorem. By solving the latter problem -- which had no previously known solution even in classical information theory -- we thus compute the optimal asymptotic error exponent of entanglement distillation. We show this asymptotic solution to be given by the reverse relative entropy of entanglement, a single-letter quantity that can be evaluated using only a single copy of a quantum state, which is a unique feature among operational measures of entanglement. Altogether, we thus demonstrate a measure of entanglement that admits a direct operational interpretation as the optimal asymptotic rate of an important entanglement manipulation protocol while enjoying an exact, single-letter formula.