NSF-NSERC: Building a two-qubit controlled phase gate using laterally coupled semiconductor quantum dots

量子比特或量子位是量子科学和信息处理的基本组成部分。生成、处理和存储量子位的能力为量子信息科学和技术奠定了基础。该合作项目旨在研究半导体系统中两个量子位之间的相互作用，这是迄今为止信息处理、通信和存储的首选技术。半导体量子点是一种类似原子的人造结构，已被证明可以有效地产生和存储量子比特。此外，量子点量子比特可以与光无缝连接，远距离传输量子信息。因此，人们对探索量子点量子比特作为未来量子计算或通信系统的基本构建块产生了极大的兴趣。一个关键但缺失的环节是确定地处理两个量子点量子比特的能力。该项目旨在通过在附近创建两个位置控制的量子点并利用它们之间的相互作用来填补技术空白。该项目的积极成果不仅可以提高使用半导体系统进行量子信息科学与工程的知识，而且可以培养训练有素的工程师和科学家。半导体量子点中孤立电子自旋具有鲁棒相干性；因此，它们是固态量子系统的有希望的量子比特候选者。异质结构量子点自旋可以很容易地与光子量子比特接口，使它们在量子网络应用中特别有吸引力。在异质结构量子点中实现单量子比特操作方面取得了重大进展。进一步扩展到双量子比特门的能力对于实现量子计算功能至关重要，例如，量子链路中的纠缠交换和大规模纠缠簇态的生成。提出的研究重点是改进异质结构量子点中的自旋自旋门。虽然最近在垂直堆叠量子点结构中报道了一个实验性的概念验证双量子位自旋门。然而，由两个电子自旋之间的交换相互作用介导的耦合不是短暂的，限制了它对通用计算需求的适用性。在两个量子点之间精确排列能级也存在实际的挑战，因为它们共享一条共同的电路径。本研究旨在通过远程和按需库仑相互作用耦合的横向定位量子点来消除这些问题。该研究建立在最近两个横向定位的氮化半导体量子点之间的自旋自旋门的理论论证的基础上，重点是推动iii族氮化量子点生长的边界，实验证明了一个可靠的过程，可以产生具有不同点间距的横向定位量子点对，点之间的瞬态库仑耦合，以及两个电子自旋之间的受控相门。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Quantum bit or qubit is the basic building block for quantum science and information processing. The ability to generate, process, and store qubits establishes the foundation for quantum information science and technologies. This collaborative project aims to investigate the interaction between two qubits in a semiconductor system which has been the technology of choice to this date for information processing, communication, and storage. Semiconductor quantum dots are atom-like man-made structures that have been shown to generate and store qubits efficiently. Moreover, quantum dot qubits can seamlessly interface with light to transmit quantum information over a distance. As a result, there have been tremendous interests in exploring quantum dot qubits as the fundamental building block for a future quantum computing or communication system. One critical yet missing link is the ability to process two quantum dot qubits deterministically. This project aims to fill the technological gap by creating two site-controlled quantum dots in a proximity and exploiting the interaction between them. Positive results of this project not only can advance the knowledge of using a semiconductor system for quantum information science and engineering but also develop highly trained engineers and scientists. Isolated electron spins in semiconductor quantum dots have robust coherence; therefore, they are promising qubit candidates for a solid-state quantum system. Heterostructure quantum dot spins can be easily interfaced with photonic qubits, making them especially attractive for quantum network applications. Significant advances have been made in implementing single-qubit operations in heterostructural quantum dots. Further extension of the ability to a two-qubit gate is crucial to enable quantum computational functionalities, e.g., entanglement swapping in a quantum link and generation of the large-scale entangled cluster state. The proposed research focuses on improving a quantum spin-spin gate in heterostructural quantum dots. While an experimental proof-of-concept two-qubit spin gate has been reported recently in a vertically stacked quantum dot structure. However, the coupling mediated by the exchange interaction between two electron spins is not transient, limiting its applicability to general-purpose computational needs. There are also practical challenges in precisely aligning the energy levels between the two quantum dots, which share a common electrical path. This research aims to eliminate these issues by using laterally positioned quantum dots coupled via long-range and on-demand Coulomb interaction. The proposed study builds upon the recent theoretical demonstration of a spin-spin gate between two laterally positioned nitride semiconductor quantum dots, focusing on pushing the boundary of group III-nitride quantum dot growth to experimentally demonstrate a reliable process to create laterally positioned quantum dot pairs with a varying interdot spacing, a transient Coulomb coupling between the dots, and a controlled phase gate between two electron spins.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.