Short-Time Decoherence and Deviation from Pure Quantum States

In systems considered for quantum computing, i.e., for control of quantum dynamics with the goal of processing information coherently, decoherence and deviation from pure quantum states, are the main obstacles to fault-tolerant error correction. At low temperatures, usually assumed in quantum computing designs, some of the accepted approaches to evaluation of relaxation mechanisms break down. We develop a new general formalism for estimation of decoherence at short times, appropriate for evaluation of quantum computing architectures.