Improving fidelities of spin ensemble qubits through exchange coupling*

Qubits consisting of numerous distant and uncoupled spins, such as rare earth (RE) or transition metal (TM) ions embedded in a matrix, can potentially offer a superior read-out signal-to-noise ratio. This advantage holds provided that the spatial variance in the control field, which modulates the spins, does not lead to substantial dephasing¹. If these spins are positioned in closer proximity, resulting in RE or TM ions deposited closely, significant exchange coupling may occur. This could counteract dephasing, thereby extending coherence times. Nevertheless, the enhanced proximity also intensifies dipole interactions, potentially reducing coherence times². By judiciously choosing a spin system, our work aims to determine the optimal spacing to balance the adverse effects of dipolar interaction-induced dephasing against the favorable influence of the Heisenberg exchange interaction that favors aligned spin states. To dissect the interplay between exchange interaction and dipole coupling in spin-qubit gates, we simulate the dynamics of a spin ensemble to comprehend the parameter space required to achieve high-fidelity single qubit gates, considering dipole, exchange coupling, anisotropy, and Zeeman terms.

1. Comm Phy 5, 284 no. 1 (2022).

2. Pro. of the National Academy of Sciences 119.15 (2022).