In a general aspect, a quantum control device includes a substrate having a substrate surface. An insulator layer is disposed over the substrate surface and defines a cavity. The insulator layer includes an insulator surface that defines an opening to the cavity. The quantum control device also includes a field-responsive layer over the insulator surface. The field-responsive layer includes a target region that resides over the opening to the cavity. The quantum control device additionally includes a projection extending from the substrate into the cavity and terminating at a tip. The projection is configured to produce an electric field that interacts with a quantum state in the target region. The tip resides in the cavity and configured to concentrate the electric field produced by the projection.