Laser cooling and spin resonance of a single spin in a quantum dot

量子点中单自旋的激光冷却和自旋共振

• 批准号: EP/E037992/1
• 负责人: Brian Gerardot
• 金额: $ 47.49万
• 依托单位: Heriot-Watt University
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2007
• 资助国家: 英国
• 起止时间: 2007 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FE037992%2F1
• 关键词: Laser; cooling; spin; resonance; single; Laser; cooling; spin; resonance; single

An electron has in addition to its charge a spin, a quantum mechanical property. For some years, proposals have been made to design electronic devices which operate by manipulating electronic spin rather than charge. The ultimate limit is to probe and manipulate individual electron spins. A single spin can exist in an arbitrary superposition of spin states, spin-up and spin-down, and more than one spin can exist in entangled states, properties that are potentially of enormous benefit in cryptography and computing. Clearly though, it is extremely challenging to manipulate the fragile quantum states of just a single spin. This proposal attempts to achieve this by first trapping a single electron in a quantum dot, a nano-structured semiconductor, and second, performing the manipulation with a highly coherent laser. The goal is not only to develop basic capabilities in initializing a single spin into a quantum state of choice but also to understand the fundamental way an electron spin interacts with its complex host environment. These interactions include tunneling interactions with mobile electron spins, highly dot and magnetic field dependent phonon processes, and interactions with the nuclear spins of the host material, and there may be others.The first challenge is to address just a single electron. This can be achieved by exploiting the Coulomb repulsion of two electrons. In the so-called Coulomb blockade regime, just one electron occupies the quantum dot; a second electron is prohibited from entering the dot by the large Coulomb repulsion between the two electrons. In a nano-sized quantum dot, for instance one made in GaAs with self-assembly, the Coulomb blockade is very pronounced and is completely established at 4.2 K, a low but easily-accessed temperature. The second challenge is to manipulate the spin. This is a highly non-trivial task. This proposal aims to achieve this by exploiting the strong optical transition across the fundamental gap of a semiconductor. The optical transition will be driven using the highly coherent output of a laser whose frequency is tuned to the resonance of the quantum dot. Crucially, a semiconductor quantum dot has well-defined selection rules. A spin up electron interacts with a laser photon with right-handed circular polarization but not with a photon with left-handed circular polarization, and vice versa. This enables schemes to be dreamt up whereby in a magnetic field, an electron spin is projected into either the spin up or spin down states by pumping the spin with the laser; and without a magnetic field, there is a strong possibility that the spin can be projected into an arbitrary superposition simply by controlling the laser polarization. In fact, continuous pumping with the laser should prevent the spin from relaxing such that a spin state can be potentially maintained for long periods of time. Spin resonance, the magnetic dipole transition of an electron spin, is a well established technique for probing spins. It is invariably carried out on a huge number of electrons in order to boost the signal. The proposal here is to perform spin resonance on just one electron. The crucial idea is to use the optical response of the quantum dot as a detector for spin resonance as this offers both a huge amplification - the absorption of a microwave photon results in the absorption of an optical photon - and a massive reduction in spatial resolution - the probing volume is determined by the small optical wavelength and not by the large microwave wavelength. Finally, the proposal addresses the question how to measure the spin state of a single electron by monitoring the resonance fluorescence, the spontaneous emission generated by the resonant laser.

电子还具有自旋，量子机械性能。几年来，已经提出了设计通过操纵电子旋转而不是电荷来操作的电子设备的建议。最终的极限是探测和操纵单个电子旋转。单个旋转可以存在于自旋状态，旋转和旋转的任意叠加中，并且在纠缠状态中可以存在多个自旋，这可能在加密和计算中具有巨大益处。显然，操纵仅单个旋转的脆弱量子状态是极具挑战性的。该建议试图通过首先将单个电子捕获在量子点，纳米结构的半导体中，其次是用高度连贯的激光器进行操作。该目标不仅是在将单个旋转初始化为选择的量子状态的基本功能中，而且还要了解电子旋转与其复杂宿主环境相互作用的基本方式。这些相互作用包括与移动电子旋转的隧道相互作用，高度点和磁场依赖性的声子过程以及与宿主材料的核自旋相互作用，并且可能还有其他。第一个挑战是仅解决单个电子。这可以通过利用两个电子的库仑排斥来实现。在所谓的库仑封锁状态下，只有一个电子占据量子点。禁止第二个电子通过两个电子之间的大库仑排斥进入点。在纳米尺寸的量子点中，例如，一个用自组装的GAA制成的量子点非常明显，并且在4.2 K处完全确定，这是一个低但易于接收的温度。第二个挑战是操纵旋转。这是一项高度平凡的任务。该提案旨在通过利用半导体的基本差距的强烈光学过渡来实现这一目标。光学转变将使用激光的高度连贯输出进行驱动，该激光器的频率调整为量子点的共振。至关重要的是，半导体量子点具有明确定义的选择规则。旋转电子与右手圆极化的激光光子相互作用，但与左手圆极化的光子不相互作用，反之亦然。这使得方案可以在磁场中进行梦想，通过用激光泵送旋转，将电子自旋投射到向上或向下旋转状态。没有磁场，只需通过控制激光极化即可将旋转投射到任意叠加中。实际上，连续泵送激光器应防止自旋放松，以便可以长时间保持自旋状态。自旋共振是电子自旋的磁偶极转变，是探测旋转的良好技术。它总是在大量电子上进行，以增强信号。这里的建议是仅对一个电子进行旋转共振。关键的想法是使用量子点的光学响应作为旋转共振的检测器，因为这既可以提供巨大的放大 - 微波光子的吸收会导致光光子的吸收 - 并大量减少空间分辨率 - 探测体积由小的光学波长和非大型微波长度和无小的微波作用确定。最后，该提案通过监测共振激光器产生的共振发射荧光来解决如何测量单个电子的自旋状态的问题。