  We calculate that the electron states of strained self-assembled Ge/Si
quantum dots provide a convenient two-state system for electrical control. An
electronic state localized at the apex of the quantum dot is nearly degenerate
with a state localized at the base of the quantum dot. Small electric fields
shift the electronic ground state from apex-localized to base-localized, which
permits sensitive tuning of the electronic, optical and magnetic properties of
the dot. As one example, we describe how spin-spin coupling between two Ge/Si
dots can be controlled very sensitively by shifting the individual dot's
electronic ground state between apex and base.
