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陈数是表征物理系统拓扑相的不变量，最近由东京工业大学的研究人员以受控方式进行了调整。他们在电子核自旋系统（即金刚石中的氮空位中心）中实现了这一壮举，观察了从零到三的陈数。这项工作为探索奇异拓扑及其在拓扑量子信息中的应用打开了大门。
陈数是一种不变的属性（或“不变量”），它表征了各种物理系统中称为“拓扑相”的独特状态。简而言之，陈数提供了对材料内部电子行为及其集体属性的深入了解。
科学家试图通过调整陈省身数来研究不同拓扑相之间的转变，以进一步阐明物质的性质。然而，系统拓扑对外部干扰的鲁棒性使其在实验上具有挑战性。尽管已经建立了理论基础，但在凝聚态物质系统中很少通过实验观察到更高的陈数。然而，材料科学和实验技术的最新进展开辟了新的可能性。 最近，包括美国马里兰大学物理系 Walsworth 教授和日本东京工业大学电气与电子工程系 Keigo Arai 副教授在内的国际研究团队进行了探索， 与金刚石氮空位 (NV) 中心相关的电子核自旋系统中陈数的跃迁。他们的研究成果发表在 npj Quantum Information 期刊上。 “NV 中心是金刚石晶格中的缺陷，由一个氮原子和一个空晶格位点组成。该系统由于其可控的电子和核自旋自由度，为研究拓扑相提供了一个独特的平台。” 研究人员通过自旋控制微波改变控制哈密顿量（用于解决动力系统最优控制问题的函数）的参数，以操纵陈数，该数代表控制哈密顿参数球内包含的简并数。因此，可以通过调整该球体的半径和偏移来引起不同拓扑相之间的转变。
接下来，该团队采用实验技术和数值模拟相结合的方式来表征系统的最终行为，观察从零到三的陈数。此外，测量的拓扑相图与数值模拟一致，并且可以映射到相互作用的三量子位系统上。最后，研究人员证明，NV 系统可以获取更高的陈数，为探索更复杂的拓扑相铺平道路。 不过，这项工作的新颖之处不仅在于观察陈数的转变，还在于证明其可控性。在单个系统内调整陈数的能力为实际应用开辟了途径。“例如，量子反常霍尔绝缘体中的高陈数相有望实现低功耗电子产品。随着陈数的增加，普通金属电极和手性边缘通道之间的接触电阻会降低，这使其成为未来电子产品的一个有吸引力的平台 ”，Arai 博士指出。 事实上，NV中心系统内陈数的可调性为探索奇异拓扑及其在拓扑量子信息中的应用提供了令人兴奋的可能性。这可能会推动量子计量学、下一代电子学、自旋电子学和量子计算领域的发展。 Tuning the Chern number in the nitrogen-vacancy center in diamond by Tokyo Institute of Technology The Chern number, an invariant quantity that characterizes topological phases in physical systems, was recently tuned in a controlled fashion by researchers from Tokyo Tech. They achieved this feat in an electronic-nuclear spin system, namely the nitrogen-vacancy center in diamond, observing Chern numbers from zero to three. This work opens doors to exploring exotic topology and its applications in topological quantum information. The Chern number is a non-changing property (or an "invariant") that characterizes unique states called "topological phases" in various physical systems. Put simply, Chern numbers provide insights into the behavior of electrons and their collective properties inside the material. Scientists try to investigate transitions between different topological phases by tuning the Chern number to shed further light on the properties of matter. However, the robustness of the system topology to external disturbances makes it experimentally challenging. Despite an established theoretical groundwork, higher Chern numbers have rarely been observed experimentally in condensed matter systems. Nevertheless, recent advancements in materials science and experimental techniques have opened up new possibilities. Recently, an international team of researchers, including Professor Walsworth from the Department of Physics at University of Maryland in the U.S. and Associate Professor Keigo Arai from the Department of Electrical and Electronic Engineering at Tokyo Institute of Technology (Tokyo Tech) in Japan, has explored the transitions of the Chern number in an electronic-nuclear spin system associated with the nitrogen-vacancy (NV) center in diamond. Their work is published in the npj Quantum Information journal.
"The NV center, a defect in the diamond lattice, consists of a nitrogen atom coupled with a vacant lattice site. This system provides a unique platform for investigating topological phases owing to its controllable electronic and nuclear spin degrees of freedom," explains Dr. Arai. The researchers varied the parameters of a control Hamiltonian (a function used to solve a problem of optimal control for a dynamical system) through spin-control microwaves to manipulate the Chern number, which represented the number of degeneracies enclosed in a control Hamiltonian parameter sphere. Consequently, transitions between different topological phases could be induced by adjusting the radius and offset of this sphere.
The team next employed a combination of experimental techniques and numerical simulations to characterize the system's resulting behavior, observing Chern numbers from zero to three. Additionally, the measured topological phase diagram was in agreement with the numerical simulations and could be mapped onto an interacting three-qubit system. Finally, the researchers demonstrated that the NV system could enable access to even higher Chern numbers, paving the way for exploring more complex topological phases. The novelty of this work, however, lies not only in observing the transitions of the Chern number but also in demonstrating its controllability. The ability to tune the Chern number within a single system opens up avenues for practical applications. "For instance, high Chern number phases in quantum anomalous Hall insulators hold promise for low-power-consumption electronics. As the Chern number increases, the contact resistance between normal metal electrodes and chiral edge channels decreases, making it an attractive platform for future electronics," points out Dr. Arai. Indeed, the tunability of the Chern number within the NV center system offers exciting possibilities for exploring exotic topologies and their applications in topological quantum information. This could potentially advance the fields of quantum metrology, next-generation electronics, spintronics, and quantum computation.
来源 Source：https://phys.org/news/2023-09-tuning-chern-nitrogen-vacancy-center-diamond.html
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