  With improved gate calibrations reducing unitary errors, we achieve a
benchmarked single-qubit gate fidelity of 99.95% with superconducting qubits in
a circuit quantum electrodynamics system. We present a method for
distinguishing between unitary and non-unitary errors in quantum gates by
interleaving repetitions of a target gate within a randomized benchmarking
sequence. The benchmarking fidelity decays quadratically with the number of
interleaved gates for unitary errors but linearly for non-unitary, allowing us
to separate systematic coherent errors from decoherent effects. With this
protocol we show that the fidelity of the gates is not limited by unitary
errors, but by another drive-activated source of decoherence such as amplitude
fluctuations.
