(color online) Fundamental diagrams for different types of qubits. Charge stability diagrams are depicted by reading the information of charge occupancy as a function of gate voltage, in which ‘ ’ (‘ ’) denote the gate that control the left (right) dot of a double quantum dot or the leftmost (rightmost) dot of a triple quantum dot, while ‘ ’ (‘ ’) denote the barrier (plunger) gate that controls the single quantum dot. Energy level spectrums are depicted by analyzing the energy level as a function of detuning ε or the magnetic field .