Figure 2.

Spin-1/2 qubit and singlet–triplet qubit. (a) and (b) are diagrams showing the process of control and readout based on spin-selective tunneling. (a) At the stage for qubit control, both energy levels of spin-up and spin-down are under the Fermi level of drain. (b) At the stage for readout, the energy levels in the dot are tuned so that the Fermi level of drain is between the energy levels of spin-down and spin-up. (c) and (d) are diagrams showing the phenomenon of spin blockade: S(1, 1) can move to S(0, 2) while T(1, 1) cannot. (e) The probability of spin-up P_up as a function of MW burst time and frequency detuning. (Adapted from [17,18].) (f) Sequence fidelities for standard (topmost) and interleaved randomized benchmarking (annotated in the figure along with extracted fidelities). Traces are offset by an increment of 0.2 for clarity. Visibilities are within 0.72 ± 0.012. (Adapted from [17,18].) (g) Energy-level spectrum of two spin states in a DQD as a function of detuning . A magnetic field splits the triplet states by the Zeeman energy E_z and the exchange interaction splits S and T₀ by . (h) Singlet probability as a function of exchange-pulse duration and detuning . (Adapted from [68].) (i) Bloch-sphere representations of state evolution in the case E_z (top) and E_z (bottom). (Adapted from [68].)