  We demonstrate that an $n^+/i/n^+$ junction is the most suitable candidate
for electronic control and readout of qubit states in quantum computing systems
based on shallow impurities. The signature of this system is that the
$n^+-$regions serve as metallic electrodes separated form the $i-$region by a
self-induced barrier (internal workfunction). The $n^+/i/n^+$ system mimics the
properties of a metal-vacuum-metal junction with the qubit (impurity atom)
placed in a ``vacuum'' $i$-region between two ``metallic'' $n^+$ electrodes. We
will show that the self-induced barrier exists in a sufficiently wide range of
the concentration of dopants in the $n^+$-semiconductor (e.g. up to $10^{21}$
cm$^{-3}$ for Si) and its height can be controlled by tuning the doping level.
A shallow donor placed in a vacuum $i$-region will be populated with one
electron in equilibrium. In the case of Li donor in Si the $n^+$-electrodes
will be used for a precision placement of the Li atom during the growth
process; for voltage control and manipulation of the qubit states; and for a
qubit readout by means of the optically stimulated resonant tunnelling. Another
important feature of our system is that the qubit states (first two lowest
energy levels of Li in Si) are separated by an energy gap from a continuum of
the many-body states of the controlling electrodes.
