| I show that the decoherence in a quantum memory based on degenerate two--level atoms interacting with a bosonic heat bath is governed by a generalized Hamming distance (called ``decoherence metric'') between the quantum code words, with a time dependent metric tensor that is specific for the heat bath. The decoherence metric allows for the complete characterization of the decoherence of all possible superpositions of code-words, and for an optimization of the over-all decoherence. The theory is applied to two-level atoms in an optical lattice or a cold gas interacting with black body radiation. I show that additional atoms which are not accounted for can substantially increase and even dominate the decoherence process. |
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