Press David, Ladd Thaddeus D, Zhang Bingyang, Yamamoto Yoshihisa
E. L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA.
Nature. 2008 Nov 13;456(7219):218-21. doi: 10.1038/nature07530.
A basic requirement for quantum information processing systems is the ability to completely control the state of a single qubit. For qubits based on electron spin, a universal single-qubit gate is realized by a rotation of the spin by any angle about an arbitrary axis. Driven, coherent Rabi oscillations between two spin states can be used to demonstrate control of the rotation angle. Ramsey interference, produced by two coherent spin rotations separated by a variable time delay, demonstrates control over the axis of rotation. Full quantum control of an electron spin in a quantum dot has previously been demonstrated using resonant radio-frequency pulses that require many spin precession periods. However, optical manipulation of the spin allows quantum control on a picosecond or femtosecond timescale, permitting an arbitrary rotation to be completed within one spin precession period. Recent work in optical single-spin control has demonstrated the initialization of a spin state in a quantum dot, as well as the ultrafast manipulation of coherence in a largely unpolarized single-spin state. Here we demonstrate complete coherent control over an initialized electron spin state in a quantum dot using picosecond optical pulses. First we vary the intensity of a single optical pulse to observe over six Rabi oscillations between the two spin states; then we apply two sequential pulses to observe high-contrast Ramsey interference. Such a two-pulse sequence realizes an arbitrary single-qubit gate completed on a picosecond timescale. Along with the spin initialization and final projective measurement of the spin state, these results demonstrate a complete set of all-optical single-qubit operations.
量子信息处理系统的一个基本要求是能够完全控制单个量子比特的状态。对于基于电子自旋的量子比特，通用单比特门是通过自旋绕任意轴旋转任意角度来实现的。两个自旋态之间受驱动的相干拉比振荡可用于证明对旋转角度的控制。由可变时间延迟分隔的两次相干自旋旋转产生的拉姆齐干涉证明了对旋转轴的控制。此前已使用需要许多自旋进动周期的共振射频脉冲证明了对量子点中电子自旋的完全量子控制。然而，对自旋的光学操纵允许在皮秒或飞秒时间尺度上进行量子控制，从而在一个自旋进动周期内完成任意旋转。光学单自旋控制的最新工作已证明了量子点中自旋态的初始化，以及在很大程度上未极化的单自旋态中对相干性的超快操纵。在这里，我们使用皮秒光脉冲证明了对量子点中初始化的电子自旋态的完全相干控制。首先，我们改变单个光脉冲的强度，以观察两个自旋态之间超过六次的拉比振荡；然后我们施加两个连续脉冲以观察高对比度的拉姆齐干涉。这样的双脉冲序列实现了在皮秒时间尺度上完成的任意单比特门。连同自旋态的初始化和最终投影测量，这些结果证明了一套完整的全光单比特操作。
