  Decoherence is a fundamental obstacle to the implementation of large-scale
and low-noise quantum information processing devices. In this work, we suggest
an approach for suppressing errors by employing pre-processing and
post-processing unitary operations, which precede and follow the action of a
decoherence channel. In contrast to quantum error correction and
measurement-based methods, the suggested approach relies on specifically
designed unitary operators for a particular state without the need in ancillary
qubits or post-selection procedures. We consider the case of decoherence
channels acting on a single qubit belonging to a many-qubit state.
Pre-processing and post-processing operators can be either individual, that is
acting on the qubit effected by the decoherence channel only, or collective,
that is acting on the whole multi-qubit state. We give a classification of
possible strategies for the protection scheme, analyze them, and derive
expressions for the optimal unitary operators providing the maximal value of
the fidelity regarding initial and final states. Specifically, we demonstrate
the equivalence of the schemes where one of the unitary operations is
individual while the other is collective. We then consider the realization of
our approach for the basic decoherence models, which include single-qubit
depolarizing, dephasing, and amplitude damping channels. We also demonstrate
that the decoherence robustness of multi-qubit states for these decoherence
models is determined by the entropy of the reduced state of the qubit
undergoing the decoherence channel.
