  Errors in quantum computers are of two kinds: sudden perturbations to
isolated qubits, and slow random drifts of all the qubits. The latter may be
reduced, but not eliminated, by means of symmetrization, namely by using many
replicas of the computer, and forcing their joint quantum state to be
completely symmetric. On the other hand, isolated errors can be corrected by
quantum codewords that represent a logical qubit in a redundant way, by several
physical qubits. If one of the physical qubits is perturbed, for example if it
gets entangled with an unknown environment, there still is enough information
encoded in the other physical qubits to restore the logical qubit, and
disentangle it from the environment. The recovery procedure may consist of
unitary operations, without the need of actually identifying the error.
