Silicon has many attractive properties for quantum computing, and the quantum-dot architecture is appealing because of its controllability and scalability. However, the multiple valleys in the silicon conduction band are potentially a serious source of decoherence for spin-based quantum-dot qubits. Only when a large energy splits these valleys do we obtain well-defined and long-lived spin states appropriate for quantum computing. Here, we show that the small valley splittings observed in previous experiments on Si–SiGe heterostructures result from atomic steps at the quantum-well interface. Lateral confinement in a quantum point contact limits the electron wavefunctions to several steps, and enhances the valley splitting substantially, up to 1.5 meV. The combination of electrostatic and magnetic confinement produces a valley splitting larger than the spin splitting, which is controllable over a wide range. These results improve the outlook for realizing spin qubits with long coherence times in silicon-based devices.
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We gratefully acknowledge conversations with R. Blick. This work was supported by NSA/LPS under ARO contract number W911NF-04-1-0389, and by the National Science Foundation through the ITR programme (DMR-0325634) and the EMT programme (CCF-0523675).
The authors declare no competing financial interests.
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Goswami, S., Slinker, K., Friesen, M. et al. Controllable valley splitting in silicon quantum devices. Nature Phys 3, 41–45 (2007). https://doi.org/10.1038/nphys475
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