  We study the decoherence of a quantum computer in an environment which is
inherently correlated in time and space. We first derive the nonunitary time
evolution of the computer and environment in the presence of a stabilizer error
correction code, providing a general way to quantify decoherence for a quantum
computer. The general theory is then applied to the spin-boson model. Our
results demonstrate that effects of long-range correlations can be
systematically reduced by small changes in the error correction codes.
