  It is shown that coherent spin motion of electron-hole pairs localized in
band gap states of silicon can influence charge carrier recombination. Based on
this effect, a readout concept for silicon based solid-state spin--quantum
computers as proposed by Kane is suggested. The 31P quantum bit (qbit) is
connected via hyperfine coupling to the spin of the localized donor electron.
When a second localized and singly occupied electronic state with an energy
level deep within the band gap or close to the valence edge is in proximity, a
gate controlled exchange between the 31P nucleus and the two electronic states
can be activated that leaves the donor-deep level pair either unchanged in a
|T->-state or shifts it into a singlet state |S>. Since the donor deep level
transition is spin-dependent, the deep level becomes charged or not, depending
on the nuclear spin orientation of the donor nucleus. Thus, the state of the
qbit can be read with a sequence of light pulses and photo conductivity
measurements.
