Quantum Error Correction

⋆ It seems very unlikely that quantum computation can be realized unless there is some means of correcting the errors which will inevitably arise when physical devices are constructed to carry out such a computation. The situation is far different from that in ordinary “classical” computers in which for most purposes the probabilities of errors are so small that they can be ignored. • The absence of errors in ordinary computers is related to the fact that bits are embodied in devices which are thermodynamically irreversible: 0 and 1 correspond to local free energy minima in a thermodynamic sense. But thermodynamic irreversibility is a great enemy of quantum computing, since it tends to decohere qubits, thus introducing unwanted noise into the quantum computation. • Effective techniques for quantum error correction were first developed in 1996 by Shor, and independently by Steane. Up till then many skeptical physicists regarded quantum computing as totally impractical. With the development of error correction techniques, “totally impractical” was replaced with “extremely difficult.”