To create a large-scale quantum computer which operates reliably in
the presence of environmental noise and decoherence will almost
certainly require some sort of error control technique. One of the
most powerful techniques is the theory of fault-tolerant quantum
computation. The threshold theorem shows that a quantum computer
which is "good enough," in terms of error rate per gate or time step,
can be used, via an appropriate encoding, to perform arbitrarily long
quantum computations. However, in order for fault-tolerance to be
possible, a number of conditions must be satisfied beyond just having
good control of your quantum system. The threshold error rate below
which fault-tolerance is possible is small, but perhaps not
unachievably so. I will discuss the requirements on a system for
fault-tolerance to be possible and conditions which improve or hurt
the threshold.
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