Local Dynamics and Control of Noisy Two-Level Systems Coupled to a Central Qubit

量子相干性是使量子器件（如量子传感器和量子计算机）比相应的经典器件具有优势的基本资源。因此，相干性（或“退相干”）的丧失最终限制了任何量子器件的性能。这种相干性的损失总是由于设备与其环境的相互作用，尽管不同系统的过程细节可能非常不同，但在许多情况下，退相干的主要来源原来是由附近的量子实体产生的噪声，称为“两级系统”。这个实验项目将集中在方法和策略的发展，以减轻由于这种系统的退相干。最终目标是使量子传感器和量子热机的实现接近最大灵敏度和效率，并提高其他有前途的量子架构的性能，例如一些超导量子器件。这项工作将推进量子科学领域的知识，对基础科学和量子器件工程都有影响。这个项目的好处将通过纳入一项旨在激发未被充分代表的高中生和普通公众对科学的兴趣的外展努力而得到扩大。由于两能级系统的集成（如隧道缺陷、顺磁吸附剂或近表面状态）引起的退相干的常见缓解策略侧重于材料和表面工程，以尽量减少这些集成的密度。这个实验项目将采用一种不同的方法：以纳米级分辨率研究顺磁两能级系统的二维系综的基本动力学特性，并开发设计系综量子态的方法，以延长附近量子位的相干时间。提出的工作将集中在金刚石中的单氮空位（NV）中心量子比特，耦合到金刚石表面的局部电子自旋。NV中心将被用作量子传感器，使研究表面自旋动力学在单自旋水平。同时，NV中心也将用作单量子位冷库，使表面自旋系综的局部交叉极化成为可能。目标是促进对具有远程相互作用的无序多体量子系统微观动力学特性的基本理解，并开发耦合到中心量子比特的相互作用自旋系统的局部量子控制技术。这个项目是由化学系共同资助的。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 coherence is the fundamental resource that gives quantum devices (such as quantum sensors and quantum computers) their advantage over the corresponding classical devices. Hence, loss of coherence (or “decoherence”) ultimately limits the performance of any quantum device. This loss of coherence is always due to the interaction of the device with its environment, and although the details of the process can be very different for different systems, in many cases the main source of decoherence turns out to be the noise produced by nearby quantum entities known as “two-level systems”. This experimental project will focus on the development of methods and strategies to mitigate the decoherence due to such systems. The ultimate goal is to enable implementation of quantum sensors and quantum heat machines operating near maximum sensitivity and efficiency, and to improve the performance of other promising quantum architectures, such as some superconducting quantum devices. This work will advance knowledge in the field of quantum science, with impact on both fundamental science and on quantum device engineering. The benefits of this project will be expanded by incorporating an outreach effort designed to spark interest in science among under-represented high-school students and the general public. A common mitigation strategy for the decoherence due to ensembles of two-level systems (such as tunneling defects, paramagnetic adsorbants, or near-surface states) focuses on materials and surface engineering to try to minimize the density of these ensembles. This experimental project will pursue a different approach: investigate with nanoscale resolution the fundamental dynamical properties of a two-dimensional ensemble of paramagnetic two-level systems, and develop methods to engineer the quantum state of the ensemble, in order to extend the coherence time of a nearby qubit. The proposed work will focus on single nitrogen-vacancy (NV) center qubits in diamond, coupled to localized electronic spins on the diamond surface. NV centers will be used as quantum sensors, enabling the study of surface spin dynamics at the single-spin level. In parallel, NV centers will also be used as single-qubit cold reservoirs, enabling local cross-polarization of the surface spin ensemble. The objectives are to advance the fundamental understanding of the microscopic dynamical properties of disordered many-body quantum systems with long-range interactions, and to develop techniques for local quantum control of ensembles of interacting spin systems coupled to a central qubit. This project is being co-funded by the Division of Chemistry.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.