(color online) Experimental demonstration of ST qubits:^[52] (a) SEM image of a device for two ST qubit gate. The green arrows denote current paths for charge sensing and electrons in the left qubit is labeled LL and LR while electrons in the right qubit is labeled RL and RR. (b) Fidelity of a single qubit formed in the device of panel (a) through randomized benchmarking. From the decay an average fidelity of 98.6% can be inferred. (c) Process tomography for the two-qubit entangling gate. i and ii are real and imaginary components of the measured process matrix, respectively, while iii and iv are of the ideal process matrix. (d) Gate fidelity of the measured process matrix and most-likely completely positive process matrix (attained by maximum likelihood estimation process) and two-qubit Bell state fidelity as a function of the interaction strength between two ST qubits, yielding maximum fidelity of 90±1%, 87±%, and 93±1%, respectively. Error bars are statistical uncertainties.