E. L. Ginzton Laboratory, Stanford University, Stanford, CA 94305, USA.
Rep Prog Phys. 2013 Sep;76(9):092501. doi: 10.1088/0034-4885/76/9/092501. Epub 2013 Sep 4.
Single spins in semiconductor quantum dots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantum dot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantum dot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a 'flying' photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantum dot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantum dots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantum dot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantum dot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is entangled with the spin, and these photons are then interfered. We review recent work demonstrating entanglement between a stationary spin qubit and a flying photonic qubit. These experiments utilize the polarization- and frequency-dependent spontaneous emission from the lowest charged exciton state to single spin Zeeman sublevels.
单半导体量子点中的自旋形成了固态量子信息处理的有前途的平台。单个电子或空穴被困在量子点内时，其自旋向上和自旋向下状态可以表示一个具有相当长退相干时间的单量子位。自旋量子位可以与激子（带电激子）态光学耦合，这些激子态也被困在量子点中，这提供了一种用光学脉冲快速初始化、操纵和测量自旋状态的机制，并为固定物质量子位和“飞行”光子量子位之间的接口提供了一种机制量子通信和分布式量子信息处理。通过将量子点置于单片微腔中，可以增强自旋量子位与光的相互作用。由二维量子点阵列和平面微腔组成的整个系统可以通过现代半导体纳米制造技术构建，并为芯片尺寸可扩展的量子中继器和量子计算机提供一种途径。本文综述了用于量子信息处理的单量子点自旋的光学控制的最新实验进展。我们重点介绍了在单个量子点自旋上执行所有光学单量子位操作的完整集合的演示：初始化、任意 SU(2) 门和测量。我们回顾了由于超精细相互作用与核自旋浴，导致退相干和相位失相的机制，并展示了如何将单量子位操作组合起来执行自旋回波序列，从而将量子位退相干时间从几纳秒扩展到几微秒，比单量子位门时间长 5 个数量级以上。讨论了双量子位耦合，既可以通过附近自旋的交换耦合和光诱导的几何相位在单个芯片内进行，也可以在更长的距离上进行。如果每个自旋都可以发射与自旋纠缠的光子，并且这些光子可以相互干涉，则可以生成长程自旋纠缠。我们回顾了最近证明固定自旋量子位和飞行光子量子位之间纠缠的工作。这些实验利用最低带电激子态的偏振和频率相关的自发发射到单个自旋塞曼子能级。
