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

量子相干是使量子器件(如量子传感器和量子计算机)相对于相应的经典器件具有优势的基本资源。因此，相干性的丧失(或“消相干”)最终限制了任何量子设备的性能。这种相干性的丧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，消相干性的主要来源原来是附近的量子实体产生的噪声，即被称为“两能级系统”的噪声。这个实验项目将专注于开发方法和策略，以减轻由于这样的系统造成的消相干。最终目标是实现接近最高灵敏度和效率的量子传感器和量子热机的实现，并提高其他有前途的量子体系结构的性能，例如一些超导量子器件。这项工作将促进量子科学领域的知识，对基础科学和量子设备工程都有影响。这一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感兴趣的外展努力来扩大。对于由两能级系统(如隧道缺陷、顺磁吸附剂或近表面态)的系综引起的退相干，一种常见的缓解策略集中在材料和表面工程上，试图最小化这些系综的密度。这个实验项目将采取一种不同的方法：以纳米级的分辨率研究二维顺磁二能级系统系综的基本动力学性质，并开发方法来设计系综的量子态，以便延长附近量子比特的相干时间。拟议的工作将集中在钻石中的单氮空位(NV)中心量子比特，以及钻石表面的局域电子自旋。NV中心将被用作量子传感器，使在单自旋水平上研究表面自旋动力学成为可能。同时，NV中心也将被用作单量子比特冷库，使表面自旋系综的局部交叉极化成为可能。其目的是促进对具有长程相互作用的无序多体量子系统微观动力学性质的基本理解，并发展耦合到中心量子比特的相互作用自旋系统系综的局域量子控制技术。这个项目是由化学部共同资助的。这个奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.