Taking Control of Superconducting Quantum Electronics

Electronic circuits which exhibit quantum mechanical phenomena—superposition and entanglement, in particular—promise a new generation of computers capable of solving currently intractable problems, secure communication, and an efficient route to simulate new materials and even cosmology. One of the fundamental challenges, however, in quantum information processing is to sustain coherence over a time interval practical for performing a computation or simulation. Until now, boosting coherence has involved hardware development to minimize coupling to a dissipative environment which typically transforms a quantum superposition into a classical state. Recent advances in the development of robust quantum-noise-limited microwave amplifiers and quantum bits with lifetimes in excess of 100 microseconds have enabled the use of feedback to actively suppress decoherence. In particular, we have been able to tailor the dissipative environment, either via measurement or control pulses, to stabilize quantum superposition states and coherent oscillations as well as track the evolution of single and two qubit states. These initial experiments suggest the plausibility of high fidelity feedback as a means to generate and maintain quantum entanglement.