Fig. 1: Device stack and electron spin resonance (ESR).

a A 3D render of the global control device stack used in our experiments, including the silicon quantum nanoelectronic device (bottom, black), sapphire dielectric spacer (middle, blue) and potassium tantalate (KTO) dielectric microwave resonator (top, pink). b A schematic cross-section through the measured silicon QD device showing the 3D structure of the gates, enriched silicon-28 substrate (merged with panel c) and insulating oxide layers. c Steps for preparing the isolated mode measurements, depicted with conduction band profiles at the interface of the ²⁸Si substrate. The preparation consists of three phases: loading (i), isolation (ii) and initialisation (iii) (see text for more details). d Charge stability diagram measured in isolated mode. Four charge transitions occur while the detuning voltage ϵ is swept from −0.6 to 0.4 V, indicating that there are 4 electrons in the double dot system. Black geometric symbols (explained in panel e) show the voltage configurations used in the ESR experiments. e Pulsing scheme for the ESR measurements. For a detailed explanation, please see Methods. f Even state probability as a function of f_MW and B₀, revealing two ESR peaks that shift with magnetic field, consistent with two spin qubits occupying the double dot system. The peaks are labelled Qubit 1 and Qubit 2. An S₁₁ reflection measurement (pink circles) probed via the coaxial loop coupler is superimposed over the 2D map. The ESR pulse duration is fixed at 1.5 μs. g Even state probability as a function of B₀ when f_MW corresponds to the centre frequency of the DR resonance (dotted lines with arrows in panel f). The pulse duration is sufficiently long to make the ESR drive incoherent (25μs), causing the spin states to become completely mixed and resulting in a peak amplitude of P_even = 0.5 for both resonances.