Coherent quantum system control, i.e, qubit operation, has received much interest from a viewpoint of quantum information processing, especially quantum computing. A small-scale test-bed for a quantum computer has been demonstrated by using solution NMR. However, from the viewpoint of scalability, a solid-state quantum computer is desirable. Among the many candidates for solid-state qubits, semiconductor systems have advantages in that they use existing cutting-edge IC technologies. The coherent control of charge, spin, nuclear spin, and exciton has been studied in semiconductor systems. In this presentation, we will discuss coherent control of electron charge and nuclear spin in semiconductor systems. A semiconductor charge qubit was embedded in a coupled quantum-dot structure . In this charge qubit, electron occupation in either of the two coupled dots operates as a quantum two-level system. It is noteworthy that this charge qubit can be controlled all-electrically in semiconductor systems. We have demonstrated a modulation of the coherent oscillation frequency by electrical control of the coupling between two dots and achieved arbitrary control of pseudospin rotation on the Bloch sphere by designing the electrical pulse applied to the system . Interactions between electron and nuclear spins have been studied in semiconductor heterostructures in the fractional-quantum-Hall regime. The nuclear spin polarization was observed in the situation where different fractional-quantum-Hall states with different spin configuration coexist in the system. All electrical control has been achieved for nuclear spin polarization and relaxation . Recently, we have succeeded in controling nuclear spin polarization in a point contact regime with a mesoscopic scale . Coherent control of nuclear spin polarization has been demonstrated by flowing radio-frequency pulse current along a micro-strip line near the point contact . These experimental achievements represent the first step towards semiconductor qubit systems for quantum information processing.
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