Coherence maintained by energy-nonpreserving system-environment interactions.

We consider a total system undergoing Schr\"{o}dinger evolution, which is composed of a central system as a qubit and a large environment as a generic many-body quantum chaotic system. The extent to which the qubit may maintain coherence is studied, under qubit-environment interactions that do not preserve the energy of the qubit. For weak and intermediate system-environment interactions, expressions are derived for the long-time average of the offdiagonal elements of the reduced density matrix of the qubit, which show that these elements may possess nonnegligible values due to the interactions. The analytical predictions are tested numerically in a model with the environment as a defect Ising chain.