Fig. 3: Strong spin–photon coupling.

a Anti-crossing of the resonator and the qubit found when plotting the resonator transmission as a function of detuning ε_rel and probe frequency ω_p at a magnetic field of B = 300 mT and α = 57°. The solid white curves are the eigenstate energies from fits to a Jaynes–Cummings model (Eq. (2) in the “Methods” section. The faint double-peak structure at ε ≈ 0 is an unambiguous signature of the strong coupling regime, g > κ, γ²⁹. b, c Cross sections at the detunings indicated by colored bars in (a). The solid lines stem from a fit to input–output theory. b Double-peak structure at ω_q ~ ω₀ (see text). The larger noise floor for ω_p ~ ω₀ (gray data) is attributed to the bare resonator which is visible in spectroscopy because of a finite coupling between DQD and leads resulting in an odd DQD occupation for a short fraction of time during data acquisition. c Transmission for ω_q ≫ ω₀, corresponding to the bare resonator. d Simulation using input–output theory with the parameters extracted from the input–output fit to (b). For these measurements, given the input-power P_in = −133 dBm, the average number of photons is n < 0.25 (see the “Methods” section).