Semiconductor quantum dots in silicon are promising qubits because of long spin coherence times and their potential for scalability. However, whether qubits with fidelities above the threshold for quantum error correction can be achieved remains to be seen. We show theoretically that such high fidelities can be achieved in two types of electrically controlled double quantum dot qubits.

The singlet-triplet (ST) qubit, formed by the singlet and triplet spin states of 2 electrons in the presence of a magnetic fields, has been studied extensively in the zero angular momentum subspace. We propose a variant of this qubit that uses instead the polarized triplet state. Due to the presence of detuning sweet spots, this qubit is protected from charge noise, and possesses coherence times for both axes of rotations limited only by nuclear noise. The qubit fidelity is further improved when nuclear noise is reduced through isotopic purification, resulting in gate fidelities up to 99.9%. [Reference: C.H. Wong et. al. , Phys. Rev. B 92, 045403(2015)]

The "hybrid" quantum dot qubit is formed by three electrons in a double dot, and has practical advantages since it does not require magnetic fields and possesses GHz gate speeds. We analyze and optimize the the ac gate fidelities of this qubit, specifically focusing on decoherence caused by 1/f charge noise,. We find parameters that minimize the charge noise fluctuations of the qubit frequency, determine the optimal working points for ac gate operations that drive the detuning and tunnel coupling, and show that fidelities up to 99.5% can be achieved.