  We demonstrate new experimental procedures for measuring small errors in a
superconducting quantum bit (qubit). By carefully separating out gate and
measurement errors, we construct a complete error budget and demonstrate single
qubit gate fidelities of 0.98, limited by energy relaxation. We also introduce
a new metrology tool -- a Ramsey interference error filter -- that can measure
the occupation probability of the state $\ket{2}$ down to $10^{-4}$, a
magnitude near the fault tolerant threshold.
