Design of Quantum Information Processing Devices Utilizing Electron Spins by Electric and Magnetic Fields

从应用于量子信息处理的角度来看，我们对半导体量子点中电子自旋的控制进行了各种基础研究。1. 我们提出了一个新的自旋注入的想法，利用一个小收缩，量子点接触（QPC），制造在半导体上。既不需要磁场也不需要磁性材料。通过对存在自旋轨道相互作用的QPC输运的数值研究，我们发现(i)可以观察到电导量子化，（ii）电流在横向上是自旋极化的，（iii）在InGaAs异质结构中可以实现50%以上的自旋极化。2. 电子自旋在硅中具有较长的相干时间，与化合物半导体相比，这是量子信息处理的一个优势。在考虑电子-电子相互作用的情况下，对硅量子点中的电子态进行了数值研究。我们已经证明，离散能级是dede&more产生的，反映了传导带的多谷结构，因此，高自旋态很容易实现。3. 考虑到量子点中电子和核自旋之间的超精细相互作用，我们提出了电流驱动下核自旋的纠缠机制。我们的机制与泡利自旋阻断双量子点中的泄漏电流有关。伴随着自旋翻转的电流逐渐增加了核自旋之间的量子相关性，从而显著提高了电子的自旋翻转速率，从而提高了泄漏电流。4. 研究了电子-声子相互作用对嵌入量子点的Aharonov-Bohm环非平衡输运的消相效应。利用非平衡格林函数，我们成功地解释了在高偏置电压下，非对称形状的法诺共振变为对称形状的实验结果。5. 我们还通过铁磁金属QPC进行了电导率的数值研究，耦合量子点中量子门操作的数值模拟，单分子器件中外部电极对碳纳米管影响的研究等。少

From a viewpoint of application to quantum information processing, we have performed various basic researches to control electron spins in semiconductor quantum dots. 1. We have proposed a new idea of spin injection using a small constriction, quantum point contact (QPC), fabricated on semiconductors. Neither magnetic fields nor magnetic materials are required. By numerical studies of the transport through a QPC in the presence of spin-orbit interaction, we have shown that (i) the conductance quantization is observed, (ii) the current is spin-polarized in the transverse direction, and (iii) spin polarization of more than 50% can be realized in InGaAs heterostructures. 2. Electron spins have a long coherent time in silicon, which is an advantage for quantum information processing compared with compound semiconductors. We have numerically studied electronic states in silicon quantum dots, taking into account electron-electron interaction. We have shown that discrete energy levels are deg … More enerate reflecting a multi-valley structure of conduction band and, as 'a result, high spin states are easily realized. 3. Considering the hyperfine interaction between electron and nuclear spins in quantum dots, we have proposed an entanglement mechanism of nuclear spins driven by the electric current. Our mechanism is relevant to a leakage current in double quantum dots with Pauli spin-blockade. The current accompanied by the spin flip gradually increases a quantum correlation among nuclear spins, which markedly enhances the spin-flip rate of electrons and hence the leakage current. 4. We have investigated the dephasing effect by electron-phonon interaction on nonequilibrium-transport in an Aharonov-Bohm ring with an embedded quantum dot. Using nonequilibrium Green function, we have successfully explained the experimental result that an asymmetric shape of Fano resonance is changed to a symmetric shape under high bias voltages. 5. We have also performed numerical study of electric conductance through a QPC made of ferromagnetic metals, numerical simulation of quantum gate operations in coupled quantum dots, study of the influence of external electrodes on carbon nanotubes in single molecule devices, etc. Less