The extreme power of quantum-information processing originates from the coherence properties of quantum states, manifested in terms of entanglement between different local (physical) qubits f1g.
In recent years, a variety of physical qubits have been realized, and controlled qubit-qubit interactions demonstrated, pointing to a promising future for large scale quantum computing technology. However, the much studied environment-induced decoherence and dissipation will inevitably destroy quantum coherence, leading to unavoidable errors of physical-qubit-based quantum information.
The most remarkable difference between quantum and classical errors is due to the fact that quantum errors are
continuous, and thus cannot be simply corrected with known error correcting schemes from classical information theory. In 1995, Shor made the important discovery that a ninequbit-based logic qubit can correct arbitrary quantum errors, despite their continuous nature.