  Spin qubits in semiconductor quantum dots represent a prominent family of
solid-state qubits in the effort to build a quantum computer. They are formed
when electrons or holes are confined in a static potential well in a
semiconductor, giving them a quantized energy spectrum. The simplest spin qubit
is a single electron spin located in a quantum dot, but many additional
varieties have been developed, some containing multiple spins in multiple
quantum dots, each of which has different benefits and drawbacks. While these
spins act as simple quantum systems in many ways, they also experience complex
effects due to their semiconductor environment. They can be controlled by both
magnetic and electric fields depending on their configuration and are therefore
dephased by magnetic and electric field noise, with different types of spin
qubits having different control mechanisms and noise susceptibilities. While
initial experiments were primarily performed in gallium arsenide (GaAs) based
materials, silicon qubits have developed substantially and research on qubits
in metal-oxide-semiconductor (Si-MOS), silicon/silicon germanium (Si/SiGe)
heterostructures, and donors in silicon is also being pursued. An increasing
number of spin qubit varieties have attained error rates that are low enough to
be compatible with quantum error correction for single-qubit gates and
two-qubit gates have been performed in several with success rates, or
fidelities, of 90-95%.
