MRI/RUI: Acquisition of an Optically Detected Electron Spin Echo System for T2 Measurements in GaAs, AlGaAs, and InGaAs Layers and Quantum Wells

“基于自旋的量子计算”是一种通过操纵粒子（如半导体中的电子）的自旋来进行数值计算的方法。自旋是电子（以及电荷或质量等其他基本粒子）的固有属性，其行为在许多方面与旋转物体的角动量相似，因此得名自旋。电子的自旋有一个方向，它可以通过改变方向与周围环境相互作用。可能影响自旋的环境因素包括磁场、其他粒子的自旋以及固体材料中原子的集体振动。为了对量子计算有用，材料内部的自旋必须是可控的——也就是说，自旋不能偶然地改变方向，或者至少这种偶然变化的时间尺度必须远远大于计算操作的时间尺度。“自旋消相”或“自旋相干时间”（T2）是描述自旋集合保持指向同一方向的速率的重要参数，也是量子计算的重要参数-T2时间至少与微秒一样长，可能需要在重要的半导体砷化镓（GaAs）中实现基于自旋的量子计算。该项目提出通过光学检测自旋回波实验技术测量GaAs和相关材料中的T2自旋相干时间。本项目将研究n-GaAs及相关材料中的电子自旋相干时间（T2）。由于半导体中电子的自旋被用作量子比特的量子计算提议，对电子自旋寿命的研究及时引起了人们的兴趣。为了使基于自旋的量子计算机方案工作，它们必须在具有相对较长的自旋相干寿命的材料中实现。该项目将通过光学检测自旋回波测量T2时间。最初的实验将包括以一种新颖的方式测量时间分辨光致发光偏振，其中通过调制带有电子脉冲序列发生器的二极管激光器来施加泵浦和探针光脉冲。这提供了一种测量自旋翻转时间的方法，T2的上界。第二组实验将对样品进行磁共振，首先对电子自旋进行光学极化，然后在磁场扫过共振时（当微波以恒定频率施加时）光学检测极化的变化。第三组实验将以这种方式结合前两个实验：(1)与第一次实验一样，采用时间分辨泵浦光脉冲和探针光脉冲；(2)将微波频率和磁场固定在共振位置，通过电子脉冲发生器对微波进行调制，得到p/4和p/2相干脉冲的自旋回波序列，通过光学测量最终自旋状态与微波脉冲延迟的关系，可以推导出T2。

"Spin-based quantum computing" is a proposed method of doing numerical computations by manipulating the spin of particles such as electrons in semiconductors. Spin is an intrinsic property of electrons (and other fundamental particles) like charge or mass, and behaves in many ways similar to the angular momentum of a spinning object-hence the name. The spin of an electron has a direction associated with it, and may interact with its environment by changing its direction. Environmental factors which may affect spins include magnetic fields, the spin of other particles, and collective vibrations of atoms in a solid material. In order to be useful for quantum computing, the spins inside a material must be controllable-that is to say, the spins must not change direction accidentally, or at least the time scale of such accidental changes must be much larger than the time scale of the computing operations. The "spin dephasing" or "spin coherence time" (T2) is an important parameter to describe the rate at which a collection of spins stay pointed in the same direction, and is an important parameter for quantum computation-T2 times at least as long as microseconds are likely necessary to make possible spin-based quantum computing in the important semiconductor gallium arsenide (GaAs). The project proposes to measure T2 spin coherence times in GaAs and related materials through an optically detected spin echo experimental technique.This project will study electron spin coherence (T2) times in n-GaAs and related materials. Research on electron spin lifetimes is of timely interest due to quantum computing proposals in which the spin of electrons in semiconductors is used as a quantum bit. In order for spin-based quantum computer schemes to work, they must be implemented in materials which have relatively long spin coherence lifetimes. This project will measure T2 times via optically detected spin echoes. The initial experiments will involve measuring time resolved photoluminescence polarization in a novel way where pump and probe optical pulses are applied by modulating a diode laser with an electron pulse sequence generator. That provides a way to measure the spin-flip time, T1-an upper bound for T2. The second set of experiments will be to perform magnetic resonance on the samples, by first optically polarizing the electron spins, and then optically detecting the change in polarization while a magnetic field is swept through resonance (as microwaves are applied at a constant frequency). The third set of experiments will be to combine the first two experiments in this fashion: (1) time-resolved pump and probe optical pulses are applied as in the first experiment, and (2) the microwave frequencies and magnetic field are held fixed at the resonant position and the microwaves are modulated by the electronic pulse generator to get a spin echo sequence of coherent p/4 and p/2 pulses, from which T2 can be deduced by making an optical measurement of the final spin state as a function of microwave pulse delays.