Measuring errors in single qubit rotations by pulsed electron   paramagnetic resonance
  The ability to measure and reduce systematic errors in single-qubit logic gates is crucial when evaluating quantum computing implementations. We describe pulsed electron paramagnetic resonance (EPR) sequences that can be used to measure precisely even small systematic errors in rotations of electron-spin-based qubits. Using these sequences we obtain values for errors in rotation angle and axis for single-qubit rotations using a commercial EPR spectrometer. We conclude that errors in qubit operations by pulsed EPR are not limiting factors in the implementation of electron-spin based quantum computers. 
John J. L. Morton, Alexei M. Tyryshkin, Arzhang Ardavan, Kyriakos   Porfyrakis, S. A. Lyon, G. Andrew D. Briggs