Fig. 1

Schematic diagram of typical electrically measured spin qubit devices. Red (blue) spins and energy levels refer to electron (nuclear) spins. a A double quantum dot device defined in a Si/SiGe quantum well. Quantum dots can be defined either in accumulation mode with a global top gate as depicted in panel c, or in depletion mode using a doping layer. b Donor qubit system in depletion mode and fabricated by silicon metal-oxide-semiconductor technology (material stack in e). The spin states of a single electron are split in a magnetic field and qubit operation is obtained via an ac magnetic field that matches the associated resonance frequency ν _e as represented in d for dots and f for donors. An ac magnetic field can be realized directly by sending an ac current through a strip-line b. Alternatively, the motion of a quantum dot due to an ac electric field created by driving a nearby gate results in an effective magnetic field due to the field gradient of a nearby nanomagnet a. The donor system forms an effective two-qubit device due to the presence of a nuclear spin, that is coupled to the electron through the hyperfine interaction with strength A. The gyromagnetic ratio γ of both the quantum dot and donor system are affected by the electric field from the nearby electrostatic gates and nearby charged defects, which causes a non-uniformity between the qubits, but can also be exploited for addressability. For high-fidelity operation it is important that the qubit states are well isolated from excited states. Particularly in silicon quantum dots, a low-energy excited state can appear due to valley degeneracy, which can be lifted in energy via a large vertical electric field.⁹⁸ The quantum-point-contact (QPC) or single-electron-transistor (SET) is used to probe the number of charges on the dots. They could potentially be avoided via gate-based dispersive read-out⁵⁷