  New schemes for the nuclear spin quantum memory are proposed based on a
system composed of two electrons or one electron coupled to a single nuclear
spin in isotopically purified group IV elemental and II-VI compound
semiconductors. The qubit consists of the singlet state and one of the triplet
states of two electrons or simply of an electron spin. These systems are free
from the decoherence due to the nuclear dipole-dipole interaction and are
advantageous for the long memory time. In the case of two electrons, the
protocol for the quantum state transfer between the electron spin qubit and the
nuclear spin qubit is based on the magnetic or electric field tuning of the
singlet-triplet state crossing and on the hyperfine coupling supplemented with
a well-defined scheme to initialize the nuclear spin. In the case of a single
electron qubit, the quantum state transfer is driven by the hyperfine
interaction itself without the need of the nuclear spin initialization. Many
practical systems are considered, e.g., two electrons loaded on a Si or ZnSe
quantum dot, a single electron charged state in a Si quantum dot doped with a P
atom, a single electron charged $ ^{28}$Si quantum dot doped with an isotope
atom of $ ^{29}$Si, and a localized electron system of Si:P and ZnSe:F in the
bulk crystal. General aspects of these systems are investigated and a
comparison of merits and demerits is made between the two-electron qubit and
the single-electron qubit.
