Dynamical decoupling and dephasing in interacting two-level systems
  We implement dynamical decoupling techniques to mitigate noise and enhance the lifetime of an entangled state that is formed in a superconducting flux qubit coupled to a microscopic two-level system. By rapidly changing the qubit's transition frequency relative to the two-level system, we realize a refocusing pulse that reduces dephasing due to fluctuations in the transition frequencies, thereby improving the coherence time of the entangled state. The coupling coherence is further enhanced when applying multiple refocusing pulses, in agreement with our $1/f$ noise model. The results are applicable to any two-qubit system with transverse coupling, and they highlight the potential of decoupling techniques for improving two-qubit gate fidelities, an essential prerequisite for implementing fault-tolerant quantum computing. 
Simon Gustavsson, Fei Yan, Jonas Bylander, Fumiki Yoshihara, Yasunobu   Nakamura, Terry P. Orlando, William D. Oliver
quant-ph cond-mat.supr-con