Quantum feedback control of a superconducting qubit: Persistent Rabi   oscillations
  The act of measurement bridges the quantum and classical worlds by projecting a superposition of possible states into a single, albeit probabilistic, outcome. The time-scale of this "instantaneous" process can be stretched using weak measurements so that it takes the form of a gradual random walk towards a final state. Remarkably, the interim measurement record is sufficient to continuously track and steer the quantum state using feedback. We monitor the dynamics of a resonantly driven quantum two-level system -- a superconducting quantum bit --using a near-noiseless parametric amplifier. The high-fidelity measurement output is used to actively stabilize the phase of Rabi oscillations, enabling them to persist indefinitely. This new functionality shows promise for fighting decoherence and defines a path for continuous quantum error correction. 
R. Vijay and C. Macklin and D. H. Slichter and S. J. Weber and K. W.   Murch and R. Naik and A. N. Korotkov and I. Siddiqi
cond-mat.mes-hall cond-mat.supr-con quant-ph