  Two-qubit quantum gates play an essential role in quantum computing, whose
operation critically depends on the entanglement between two qubits. Resonantly
driven controlled-NOT (CNOT) gates based on silicon double quantum dots (DQDs)
are studied theoretically. The physical mechanisms for effective gate
modulation of the exchange coupling between two qubits are elucidated. Scaling
behaviors of the singlet-triplet energy split, gate-switching speed, and gate
fidelity are investigated as a function of the quantum dot spacing and
modulation gate voltage. It is shown that the entanglement strength and
gate-switching speed exponentially depend on the quantum dot spacing. A small
spacing of ~10nm can promise a CNOT gate delay of <1 ns and reliable gate
switching in the presence of decoherence. The results show promising
performance potential of the resonantly driven two-qubit quantum gates based on
aggressively scaled silicon DQDs.
