  The discovery of quantum error correction has greatly improved the long-term
prospects for quantum computing technology. Encoded quantum information can be
protected from errors that arise due to uncontrolled interactions with the
environment, or due to imperfect implementations of quantum logical operations.
Recovery from errors can work effectively even if occasional mistakes occur
during the recovery procedure. Furthermore, encoded quantum information can be
processed without serious propagation of errors. In principle, an arbitrarily
long quantum computation can be performed reliably, provided that the average
probability of error per gate is less than a certain critical value, the
accuracy threshold. It may be possible to incorporate intrinsic fault tolerance
into the design of quantum computing hardware, perhaps by invoking topological
Aharonov-Bohm interactions to process quantum information.
