Improving quantum gate fidelities by using a qubit to measure microwave   pulse distortions
  We present a new method for determining pulse imperfections and improving the single-gate fidelity in a superconducting qubit. By applying consecutive positive and negative $\pi$ pulses, we amplify the qubit evolution due to microwave pulse distortion, which causes the qubit state to rotate around an axis perpendicular to the intended rotation axis. Measuring these rotations as a function of pulse period allows us to reconstruct the shape of the microwave pulse arriving at the sample. Using the extracted response to predistort the input signal, we are able to improve the pulse shapes and to reach an average single-qubit gate fidelity higher than 99.8%. 
Simon Gustavsson, Olger Zwier, Jonas Bylander, Fei Yan, Fumiki   Yoshihara, Yasunobu Nakamura, Terry P. Orlando, William D. ...
quant-ph