  Gate operations in a quantum information processor are generally realized by
tailoring specific periods of free and driven evolution of a quantum system.
Unwanted environmental noise, which may in principle be distinct during these
two periods, acts to decohere the system and increase the gate error rate.
While there has been significant progress characterizing noise processes during
free evolution, the corresponding driven-evolution case is more challenging as
the noise being probed is also extant during the characterization protocol.
Here we demonstrate the noise spectroscopy (0.1 - 200 MHz) of a superconducting
flux qubit during driven evolution by using a robust spin-locking pulse
sequence to measure relaxation (T1rho) in the rotating frame. In the case of
flux noise, we resolve spectral features due to coherent fluctuators, and
further identify a signature of the 1MHz defect in a time-domain spin-echo
experiment. The driven-evolution noise spectroscopy complements free-evolution
methods, enabling the means to characterize and distinguish various noise
processes relevant for universal quantum control.
