  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
these valleys are split by a large energy does one 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 electronic 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.
