  Semiconductor quantum dots in silicon are promising qubits because of long
spin coherence times and their potential for scalability. However, such qubits
with complete electrical control and fidelities above the threshold for quantum
error correction have not yet been achieved. We show theoretically that the
threshold fidelity can be achieved with ac gate operation of the quantum dot
hybrid qubit. Formed by three electrons in a double dot, this qubit is
electrically controlled, does not require magnetic fields, and runs at GHz gate
speeds. We analyze the decoherence caused by 1/f charge noise in this qubit,
find the parameter regime for tunnel couplings and detuning that minimize the
charge noise dependence in the qubit frequency, and determine the optimal
working points for ac gate operations that drive the detuning and tunnel
coupling.
