  The Quantum Computer Condition (QCC) provides a rigorous and completely
general framework for carrying out analyses of questions pertaining to
fault-tolerance in quantum computers. In this paper we apply the QCC to the
problem of fluctuations and systematic errors in the values of characteristic
parameters in realistic systems. We show that fault-tolerant quantum
computation is possible despite variations in these parameters. We also use the
QCC to explicitly show that reliable classical computation can be carried out
using as input the results of fault-tolerant, but imperfect, quantum
computation. Finally, we consider the advantages and disadvantages of the
superoperator and diamond norms in connection with application of the QCC to
various quantum information-theoretic problems.
