Self-consistent measurement and state tomography of an exchange-only spin qubit

• Published in: Nature nanotechnology Vol. 8; no. 9; pp. 654 - 659
• Main Authors: Medford, J., Beil, J., Taylor, J. M., Bartlett, S. D., Doherty, A. C., Rashba, E. I., DiVincenzo, D. P., Lu, H., Gossard, A. C., Marcus, C. M.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.09.2013; Nature Publishing Group
• Subjects: 639/766/483/2802; 639/925/927/481; Control systems; Electric fields; Electrons; Estimators; Exchange; Leakage; Materials Science; Nanotechnology; Nanotechnology and Microengineering; Quantum dots; Qubits (quantum computing); Tomography; United States > US; California
• ISSN: 1748-3387; 1748-3395
• DOI: 10.1038/nnano.2013.168

Quantum-dot spin qubits characteristically use oscillating magnetic or electric fields, or quasi-static Zeeman field gradients, to realize full qubit control. For the case of three confined electrons, exchange interaction between two pairs allows qubit rotation around two axes, hence full control, using only electrostatic gates. Here, we report initialization, full control, and single-shot readout of a three-electron exchange-driven spin qubit. Control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in less than 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. Full control by electric gates can be accomplished in exchange-driven spin qubits.