  We formulate a scheme for fault-tolerant quantum computation that works
effectively against highly biased noise, where dephasing is far stronger than
all other types of noise. In our scheme, the fundamental operations performed
by the quantum computer are single-qubit preparations, single-qubit
measurements, and conditional-phase (CPHASE) gates, where the noise in the
CPHASE gates is biased. We show that the accuracy threshold for quantum
computation can be improved by exploiting this noise asymmetry; e.g., if
dephasing dominates all other types of noise in the CPHASE gates by four orders
of magnitude, we find a rigorous lower bound on the accuracy threshold higher
by a factor of five than for the case of unbiased noise.
