Qubits based on semiconductor quantum dots

Semiconductor quantum dots are promising hosts for qubits to build a quantum processor. In the last twenty years, intensive researches have been carried out and diverse kinds of qubits based on different types of semiconductor quantum dots were developed. Recent advances prove high fidelity single and two qubit gates, and even prototype quantum algorithms.These breakthroughs motivate further research on realizing a fault tolerant quantum computer. In this paper we review the main principles of various semiconductor quantum dot based qubits and the latest associated experimental results. Finally the future trends of those qubits will be discussed.     

基于重叠栅工艺的硅量子点阵列的制备与调控

利用半导体量子点阵列结构实现近邻耦合是规模化扩展自旋量子比特的主要方案之一. 随着量子点数目的增加, 量子点阵列器件的制作工艺及参数调控均愈加复杂. 本文介绍了一种重叠栅工艺结构, 利用多层相互重叠且具有不同功能的栅极定义量子点, 制作出结构紧凑、 调控性好的量子点阵列器件, 解决了工艺扩展的难题. 此外,本文发展了一套高效可靠的调控方法, 按顺序逐个添加量子点并建立虚拟电极, 实现了对量子点参数的独立控制,并且能够高效且独立地调控各量子点中的电子数目, 克服了大规模量子点阵列中电压参数配置的困难. 这些方法为未来实现大规模自旋比特阵列提供了一种标准化的方案.     

Optoelectronic properties of bottom gate-defined in-plane monolayer WSe_2 p–n junction

Monolayer transition-metal dichalcogenides(TMDs) are considered to be fantastic building blocks for a wide variety of optical and optoelectronic devices such as sensors, photodetectors, and quantum emitters, owing to their direct band gap,transparency, and mechanical flexibility. The core element of many conventional electronic and optoelectronic devices is the p–n junction, in which the p-and n-types of the semiconductor are formed by chemical doping in different regions.Here, we report a series of optoelectronic studies on a monolayer WSe_2 in-plane p–n photodetector, demonstrating a lowpower dissipation by showing an ambipolar behavior with a reduced threshold voltage by a factor of two compared with the previous results on a lateral electrostatically doped WSe_2 p–n junction. The fabrication of the device is based on a polycarbonates(PC) transfer technique and hence no electron-beam exposure induced damage to the monolayer WSe_2 is expected. Upon optical excitation, the photodetector demonstrates a photoresponsivity of 0.12 mA·W~(-1) and a maximum external quantum efficiency of 0.03%. Our study provides an alternative platform for a flexible and transparent twodimensional photodetector, from which we expect to further promote the development of next-generation optoelectronic devices.     

Gap plasmon-enhanced photoluminescence of monolayer MoS_2 in hybrid nanostructure

Monolayer transition-metal dichalcogenides(TMDs) have attracted a lot of attention for their applications in optics and optoelectronics.Molybdenum disulfide(MoS_2),as one of those important materials,has been widely investigated due to its direct band gap and photoluminescence(PL) in visible range.Owing to the fact that the monolayer MoS_2 suffers low light absorption and emission,surface plasmon polaritons(SPPs) are used to enhance both the excitation and emission efficiencies.Here,we demonstrate that the PL of MoS_2 sandwiched between 200-nm-diameter gold nanoparticle(Au NP) and 150-nm-thick gold film is improved by more than 4 times compared with bare MoS_2 sample.This study shows that gap plasmons can possess more optical and optoelectronic applications incorporating with many other emerging two-dimensional materials.     

自发参量下转化光子对的轨道角动量

自发参量下转换过程中产生的光子对的轨道角动量是纠缠的.我们计算了薄晶体产生的光子对轨道角动量的相对振幅.结果显示相对振幅依赖于两个拉盖尔系数l和p.我们还讨论了实验中用来模式分析的全息片和单模光纤的影响.我们认为下转换光子对轨道角动量的无限维在实际中只有有限维是可用的.     

Form of Scaling Function in Quantum Hall Plateau Transitions

We have measured the temperature dependence of the resistance Rxx and Rxy of a two-dimensional electron system in the regime of the quantum Hall plateau transition. We observe for our sample a considerable large critical exponent κ～ 0.66 - 0.77, which may be due to the dominant electron-phonon scattering. Further we find a simple exponential form of Rxx = Rc exp（-s） in agreement with the theoretically proposed universal scaling function.     

Effect of Quantum Point Contact Measurement on Electron Spin State in Quantum Dots

We study the time evolution of two electron spin states in a double quantum-dot system, which includes a nearby quantum point contact （QPC） as a measurement device. We find that the QPC measurement induced decoherence is in the microsecond timescale. We also find that the enhanced QPC measurement will trap the system in its initial spin states, which is consistent with the quantum Zeno effect.     

Localization Exponent for the Second Landau Level in the Quantum Hall Effect

At temperature above 1 K, we measured the temperature dependence of the longitudinal and Hall resistivity ρxx,ρxy in the regime of the quantum Hall plateau-to-plateau transitions. The localization exponent v is extracted with an approach based on the variable range hopping theory. We find the quantity v ≈ 2.3 at the second Landau level, which is proven to be accurately universal.     

Electron States in Parallel Double Quantum Dots with a Tunable Inter-Dot Coupling

We study the electron states on lateral double quantum dots coupled in parallel. The charge stability diagrams are given in terms of the gate voltages of both dots. We discover that the two electron states translate from separated states to coupled states continuously by increasing the inter-dot coupling strength. Our results demonstrate that the parallel-quantum-dot tunability bodes well for future quantum computing applications.