  Quantum correlations between two systems can be impaired by the presence of
an environment, as it can make systems decohere. We wish to evaluate how much
coherence has been irreversibly lost. To do so, we study two qubits which leak
correlations into an arbitrarily large environment and we find that our ability
to steer one of them to the most coherent set of states possible by acting on
the other is given by the sub-fidelity of two conditional states of the
environment. Sub-fidelity is a quantity that was already known, but that until
now lacked an operational interpretation. Furthermore, we conjecture the
existence of a family of measures, of which fidelity and sub-fidelity are the
first two, which measure the irreversibility of decoherence as we gain access
to more and more degrees of freedom of the enviroment.
