  We present an approach for entangling electron spin qubits localized on
spatially separated impurity atoms or quantum dots via a multi-electron,
two-level quantum dot. The effective exchange interaction mediated by the dot
can be understood as the simplest manifestation of
Ruderman-Kittel-Kasuya-Yosida exchange, and can be manipulated through gate
voltage control of level splittings and tunneling amplitudes within the system.
This provides both a high degree of tuneability and a means for realizing
high-fidelity two-qubit gates between spatially separated spins, yielding an
experimentally accessible method of coupling donor electron spins in silicon
via a hybrid impurity-dot system.
