Nanometre-scale nuclear-spin device for quantum information processing

We have developed semiconductor point contact devices in which nuclear spins in a nanoscale region are coherently controlled by all-electrical methods. Different from the standard nuclear-magnetic resonance technique, the longitudinal magnetization of nuclear spins is directly detected by measuring resistance, resulting in ultra-sensitive detection of the microscopic quantity of nuclear spins. All possible coherent oscillations have been successfully demonstrated between two levels from four nuclear spin states of I ≤ 3/2 nuclei. Quantum information processing is discussed based on two fictitious qubits of an I ≤ 3/2 system and methods are described for performing arbitrary logical gates both on one and two qubits. A scheme for quantum state tomography based on M_z-detection is also proposed. As the starting point of quantum manipulations, we have experimentally prepared the effective pure states for the I ≤ 3/2 nuclear spin system.