TY - JOUR T1 - The Resonant Exchange Qubit JF - Physical Review Letters Y1 - 2013 A1 - J. Medford A1 - J. Beil A1 - J. M. Taylor A1 - E. I. Rashba A1 - H. Lu A1 - A. C. Gossard A1 - C. M. Marcus AB - We introduce a solid-state qubit in which exchange interactions among confined electrons provide both the static longitudinal field and the oscillatory transverse field, allowing rapid and full qubit control via rf gate-voltage pulses. We demonstrate two-axis control at a detuning sweet-spot, where leakage due to hyperfine coupling is suppressed by the large exchange gap. A {\pi}/2-gate time of 2.5 ns and a coherence time of 19 {\mu}s, using multi-pulse echo, are also demonstrated. Model calculations that include effects of hyperfine noise are in excellent quantitative agreement with experiment. VL - 111 UR - http://arxiv.org/abs/1304.3413v2 CP - 5 J1 - Phys. Rev. Lett. U5 - 10.1103/PhysRevLett.111.050501 ER - TY - JOUR T1 - Self-Consistent Measurement and State Tomography of an Exchange-Only Spin Qubit JF - Nature Nanotechnology Y1 - 2013 A1 - J. Medford A1 - J. Beil A1 - J. M. Taylor A1 - S. D. Bartlett A1 - A. C. Doherty A1 - E. I. Rashba A1 - D. P. DiVincenzo A1 - H. Lu A1 - A. C. Gossard A1 - C. M. Marcus AB - We report initialization, complete electrical control, and single-shot readout of an exchange-only spin qubit. Full control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in under 2 ns. Measurement and state tomography are performed using a maximum-likelihood estimator method, allowing decoherence, leakage out of the qubit state space, and measurement fidelity to be quantified. The methods developed here are generally applicable to systems with state leakage, noisy measurements, and non-orthogonal control axes. VL - 8 U4 - 654 - 659 UR - http://arxiv.org/abs/1302.1933v1 CP - 9 J1 - Nature Nanotech U5 - 10.1038/nnano.2013.168 ER -