  We consider charge qubits based on shallow donor electron states in silicon
and coupled quantum dots in GaAs. Specifically, we study the feasibility of
P$_2^+$ charge qubits in Si, focusing on single qubit properties in terms of
tunnel coupling between the two phosphorus donors and qubit decoherence caused
by electron-phonon interaction. By taking into consideration the multi-valley
structure of the Si conduction band, we show that inter-valley quantum
interference has important consequences for single-qubit operations of P$_2^+$
charge qubits. In particular, the valley interference leads to a
tunnel-coupling strength distribution centered around zero. On the other hand,
we find that the Si bandstructure does not dramatically affect the
electron-phonon coupling and consequently, qubit coherence. We also critically
compare charge qubit properties for Si:P$_2^+$ and GaAs double quantum dot
quantum computer architectures.
