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Dissipation Enhanced Coherence in Superconducting Quantum Bits


A dissipative environment usually transforms a qua ntum superposition into a classical state. Recent advances in superconducting circuits--the development of robust quantum-noise-limited microwave amplifiers and quantum bits with lifetimes in excess of 100ms--have enabled the use of quantum feedback to actively suppress decoherence. We discuss experiments in which microwave pulses alter the circuit environment to autonomously cool the system to any coherent superposition of ground and excited states.  In addition, we also realize weak measurements of the qubit state to implement real-time feedback. Here, the dominant dephasing is measurement induced and the information extracted is used to generate Rabi oscillations which persist indefinitely.