EAGER: Deterministic Placement of Qubits in Cavities for Strongly Coupled Quantum Repeaters

Harvard University, Cambridge MA

Investigators

Abstract

This EAGER project on Deterministic Placement of Qubits in Cavities for Strongly-Coupled Quantum Repeaters studies ways of optimally placing qubits within specially engineered environments (cavities) that will allow us to store, amplify and control the release of information contained by the qubit. Qubits, corresponding to quantum mechanical states, are the fundamental units of information in system where the rules of quantum mechanics explicitly govern the exchange, storage and transmission of information. Qubits within quantum systems hold great promise for faster, more secure processing and transmission of complex information, but there are profound, fundamental challenges that limit the longevity, storage and signal strength of qubit information. The use of specially-designed cavities can result in substantial amplification of qubit optical signals, allow storage of qubit information and provide a means of controlled transmission of the qubit information: such strongly coupled qubit-cavity systems can provide an important building block of larger-scale quantum information systems. A major challenge here lies in the optimal placement of the qubit within the cavity: the efficacy of the cavity can fall off dramatically with misalignments of only several atomic lattice distances. The research proposed here uses a semiconductor platform, developing sensitive means of fine-tuning the placement of atomic-scale qubits within the cavity, and using the amplified signals from the qubit-cavity system itself to guide the process. The knowledge gained from this research can have a profound and wide-ranging impact on a variety of qubit-cavity systems, bringing us much closer to achieving robust, scalable quantum information systems on a semiconductor "chip". Technical Description: The proposed EAGER project on Deterministic Placement of Qubits in Cavities for Strongly-Coupled Quantum Repeaters will build on exciting results observed in prior experiments involving Silicon Vacancies in 4H-SiC nanobridge cavities. These experiments suggest the possibility of deterministically placing defect-based qubits in spatial overlap with the maxima of the electromagnetic fields (modes) of the cavity, one of the greatest challenges in achieving strong coupling of qubit and cavity. The work of this project develops and tests a number of strategies to achieve better control over the diffusive motion of these defect-based qubits. To understand and interpret the placement process, a detailed understanding is required of the energy landscape of the qubit, and its sensitivity to thermal, electronic and strain variations; thus a collaborative effort between experiment and theory is mandatory for success. Therefore, the proposed work closely couples experiments, complemented and guided by theoretical simulations. The research team evaluates thermal control of the diffusion process, as well as the more localized and potentially better-controlled radiation enhanced diffusion processes. The team also explores the possibility of a self-aligned process, using the high field (modal) region of the cavity itself. Achieving the correct spatial overlap of qubit to the maximum field in a cavity is perhaps the greatest challenge to attaining strong coupling. Therefore, the studies proposed here will have a significant, wide-spread impact on creating scalable quantum information systems, across a broad spectral range. The resulting strongly coupled qubit-cavity devices can serve as effective quantum repeaters that link information from discrete, spatially-separated qubits across a network. Moreover, the collaborative and closely-coupled interaction between theory and experiment represented by this team provides a richer and more complete research and education environment, as well as setting benchmarks for how such challenging problems can and should be addressed.