Irfan Siddiqi, Associate Professor of Physics, Condensed Matter Experiment, UC Berkeley
A dissipative environment usually transforms a quantum 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 100µs--have enabled the use of feedback to actively suppress decoherence. Our experimental architecture is based on a superconducting quantum bit coupled to a readout cavity. By applying microwave pulses, we modify the spectrum of quantum fluctuations experienced by the qubit to autonomously cool the system to any coherent superposition of ground and excited states. We also perform weak measurement of the qubit state to implement continuous quantum feedback. Using this technique to counteract measurement induced backaction, we demonstrate Rabi oscillations which persist indefinitely. These experiments suggest the plausibility of high fidelity measurement as a means to generate entanglement and implement error correction.