NSF-NSERC: Building a two-qubit controlled phase gate using laterally coupled semiconductor quantum dots

Quantum bit or qubit is the basic building block for quantum science and information processing. The ability to generate, process, and store qubits establishes the foundation for quantum information science and technologies. This collaborative project aims to investigate the interaction between two qubits in a semiconductor system which has been the technology of choice to this date for information processing, communication, and storage. Semiconductor quantum dots are atom-like man-made structures that have been shown to generate and store qubits efficiently. Moreover, quantum dot qubits can seamlessly interface with light to transmit quantum information over a distance. As a result, there have been tremendous interests in exploring quantum dot qubits as the fundamental building block for a future quantum computing or communication system. One critical yet missing link is the ability to process two quantum dot qubits deterministically. This project aims to fill the technological gap by creating two site-controlled quantum dots in a proximity and exploiting the interaction between them. Positive results of this project not only can advance the knowledge of using a semiconductor system for quantum information science and engineering but also develop highly trained engineers and scientists. Isolated electron spins in semiconductor quantum dots have robust coherence; therefore, they are promising qubit candidates for a solid-state quantum system. Heterostructure quantum dot spins can be easily interfaced with photonic qubits, making them especially attractive for quantum network applications. Significant advances have been made in implementing single-qubit operations in heterostructural quantum dots. Further extension of the ability to a two-qubit gate is crucial to enable quantum computational functionalities, e.g., entanglement swapping in a quantum link and generation of the large-scale entangled cluster state. The proposed research focuses on improving a quantum spin-spin gate in heterostructural quantum dots. While an experimental proof-of-concept two-qubit spin gate has been reported recently in a vertically stacked quantum dot structure. However, the coupling mediated by the exchange interaction between two electron spins is not transient, limiting its applicability to general-purpose computational needs. There are also practical challenges in precisely aligning the energy levels between the two quantum dots, which share a common electrical path. This research aims to eliminate these issues by using laterally positioned quantum dots coupled via long-range and on-demand Coulomb interaction. The proposed study builds upon the recent theoretical demonstration of a spin-spin gate between two laterally positioned nitride semiconductor quantum dots, focusing on pushing the boundary of group III-nitride quantum dot growth to experimentally demonstrate a reliable process to create laterally positioned quantum dot pairs with a varying interdot spacing, a transient Coulomb coupling between the dots, and a controlled phase gate between two electron spins. 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.