2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/93893I give a pedagogical account of Shor's nine-bit code for correcting arbitrary errors on single qubits, and I review work that determines when it is possible to maintain quantum coherence by reversing the deleterious effects of open-system quantum dynamics. The review provides an opportunity to introduce an efficient formalism for handling superoperators. I present and prove some bounds on entanglement fidelity, which might prove useful in analyses of approximate error correction.Paper based on a talk presented at the International Workshop on Macroscopic Quantum Tunneling and Coherence, Naples, Italy, June 10-13, 1998; to be published in Superconductivity; 17 pages, no figures, LaTeXQuantum Physicstext