Quantum information processing using nuclear spin orientation in a single quantum dot

在这个项目中，我们对核自旋系综的量子力学叠加态的存储应用进行了可行性研究。换句话说，我们试图阐明量子信息存储的可能性，即通过超精细相互作用将光子转换为由单电子自旋介导的核自旋系综。为此，我们必须认识到电子和核自旋翻转时的能量守恒，即外部磁场和有效核场的抵消。2006年，我们研究了单个InAlAs/GaAs量子点（QD）上核场的形成与激发光的强度和圆偏振度的函数关系。作为结果，我们观察到的核磁场作为一个函数的强度的激发光，这是圆偏振的滞后。我们可以将这种现象解释为核自旋翻转，并得出实现了外磁场与有效核场的抵消的结论。这是电子核自旋量子比特转换的必要条件。在2007年，我们将我们的研究推进到包括磁场依赖性。在增加场时，核场增加到某一阈值，在该阈值以上，核场不连续地减小。通过与理论计算结果的比较，我们发现核场完全抵消了外磁场。然而，不幸的是，这个核场是由核自旋极化的形成和破坏的平衡决定的，并不表明完美的极化或核暗态，这两者都可以用于电子核自旋量子比特转换。这表明InAlAs量子点的现状不适合量子比特转换，需要增强核自旋形成或减少自旋形变。

In this project, we did a feasibility study of memory applications of quantum mechanically superposed states of an ensemble of nuclear spins. In other words, we tried to clarify the possibility of quantum information storage using quantum bit (qubit) conversion of a photon to an ensemble of nuclear spins mediated by a single electron spin through hyperfine interaction. For this purpose, we had to realize the situation where energy at the flip-flop of electron and nuclear spins is conserved, that is, the cancellation of external magnetic field and the effective nuclear field.In 2006, we studied the formation of nuclear field as functions of intensity and degree of circular polarization of excitation light on a single InAlAs/GaAs quantum dot (QD). As a result, we observed hysteresis in the nuclear magnetic field as a function of intensity of the excitation light which is circularly polarized. We can interpret this phenomenon as a nuclear spin switching and we concluded that the cancellation of external magnetic field and the effective nuclear field was realized. This is a condition necessary for the electron nuclear spin qubit conversion.In 2007, we advanced our study to include the magnetic field dependence. Upon increasing field the nuclear field increased to a certain threshold value above which the nuclear field discontinuously decreased. Comparison with theoretical calculations tells us that the nuclear field completely cancelled the external magnetic field. Unfortunately, however, this nuclear field is determined by the balance of the formation and destruction of nuclear spin polarization, and is not indicating the perfect polarization or the nuclear dark state both of which can be used for the electron nuclear spin qubit conversion. This shows that the current status of InAlAs QD is not suitable for the qubit conversion and either enhancement of nuclear spin formation or reduction of spin deformation is needed.