High-fidelity gates in a single Josephson qubit

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 vertical bar 2 >, which is outside the computational basis, down to 10(-4), thereby confirming that our quantum system stays within the qubit manifold during single qubit logic operations.