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Abstract
Coherence in an open quantum system is degraded through its interaction with a bath.
This decoherence can be avoided by restricting the dynamics of the system to special
decoherence-free subspaces. These subspaces are usually constructed under the assumption
of spatially symmetric system-bath coupling. Here we show that decoherence-free subspaces
may appear without spatial symmetry. Instead, we consider a model of system-bath interactions
in which to first order only multiple-qubit coupling to the bath is present, with
single-qubit system-bath coupling absent. We derive necessary and sufficient conditions
for the appearance of decoherence-free states in this model, and give a number of
examples. In a sequel paper we show how to perform universal and fault tolerant quantum
computation on the decoherence-free subspaces considered in this paper.
Comments 18 pages, no figures. Major changes. Section on universal fault
tolerant computation removed. This section contained a crucial error. A new
paper [quant-ph/0007013] presents the correct analysis