  Electronic spins in Silicon (Si) are rising contenders for qubits -- the
logical unit of quantum computation-- owing to its outstanding spin coherence
properties and compatibility to standard electronics. A remarkable limitation
for spin quantum computing in Si hosts is the orbital degeneracy of this
material's conduction band, preventing the spin-1/2 states from being an
isolated two-level system. So far available samples of Si quantum wells cladded
by Ge-Si alloy barriers provide relatively small valley splitting (VS), with
the order of 1 meV or less, degrading the fidelity of qubits encoded in spin
"up" and "down" states in Si. Here, based on an atomically resolved
pseudopotential theory, we demonstrate that ordered Ge-Si layered barriers
confining a Si slab can be harnessed to enhance the VS in the active Si region
by up to one order of magnitude compared to the random alloy barriers adopted
so far. A biologically inspired genetic-algorithm search is employed to
identify magic Ge/Si layer sequences of the superlattice barriers that isolate
the electron ground state in a single valley composition with VS as large as ~9
meV. The enhanced VS is preserved with the reasonable inter-layer mixing
between different species, and is interestingly "protected" even if some larger
mixing occurs. Implementation of the optimized layer sequences of barriers,
within reach of modern superlattice growth techniques, overcomes in a practical
systematic way the main current limitations related to the orbital degeneracy,
thus providing a roadmap for reliable spin-only quantum computing in Si.
