Quantum state restoration by quantum cloning and measurement

Copying information is an elementary operation in classical information processing. However, copying seems rather different in the quantum regime. Since the discovery of the universal quantum cloning machine, much has been found from the fundamental point of view about quantum copying. But a basic question as to the utility of universal quantum cloning remains. We have considered its application in quantum state restoration by using cloning circuit for state estimation. It might be expected that classical information from the state estimation might help restore the quantum state that was disturbed during transmission. We find that the fidelity of the final state is, interestingly, independent of error probabilities inside the channel. However, this also turns out to impose a severe constraint on our original aims.