  A scheme is presented for protecting one-qubit quantum information against
decoherence due to a general environment and local exchange interactions. The
scheme operates essentially by distributing information over two pairs of
qubits and through error prevention procedures. In the scheme, quantum
information is encoded through a decoherence-free subspace for collective phase
errors and exchange errors affecting the qubits in pairs; leakage out of the
encoding space due to amplitude damping is reduced by quantum Zeno effect. In
addition, how to construct decoherence-free states for n-qubit information
against phase and exchange errors is discussed.
