  An architecture for a quantum computer is presented in which spins associated
with donors in silicon function as qubits. Quantum operations on the spins are
performed using a combination of voltages applied to gates adjacent to the
spins and radio frequency applied magnetic fields resonant with spin
transitions. Initialization and measurement of electron spins is made by
electrostatic probing of a two electron system, whose orbital configuration
must depend on the spin states of the electrons because of the Pauli Principle.
Specific devices will be discussed which perform all the necessary operations
for quantum computing, with an emphasis placed on the qualitative principles
underlying their operation.
  The likely impediments to achieving large-scale quantum computation using
this architecture will be addressed: the computer must operate at extremely low
temperature, must be fabricated from devices built with near atomic precision,
and will require extremely accurate gating operations in order to perform
quantum logic. Refinements to the computer architecture will be presented which
could remedy each of these deficiencies. I will conclude by discussing a
specific realization of the computer using Si/SiGe heterostructures into which
donors are deposited using a low energy focused ion beam.
