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

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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.