  We identify a novel mechanism to read out a single solid-state electron spin
using an all-electrical spin-to-charge conversion in a closed system. Our
scheme uses three donors and two electron spins, one spin is the qubit, the
other is a reference. The population in the third, originally ionized, donor is
monitored with an electrometer. Energy-dependent tunnelling of the reference
spin to the ionized donor is used to determine the state of the qubit. In
contrast to previous methods [e.g., B.E. Kane, Nature (London), \textbf{393},
133 (1998)] we avoid double electron occupancy of any site within the system,
thereby eliminating the possibility of unwanted electron loss from the system.
The single spin readout scheme described here is applicable to both electron
and nuclear spin based quantum computer architectures.
