Fig. 3

Electron paramagnetic resonance (EPR) spectroscopy. a Results of EPR spectroscopy for nitrogen vacancy (NV) centers in diamond under a 5.8-mT in-plane magnetic field. The qubit excitation frequency is converted into the corresponding magnetic flux. For the spectroscopy, the temperature is fixed at 20 mK to maximize the spin-polarization ratio. b EPR peak frequency as a function of the in-plane magnetic field. The magenta circles (blue triangles) correspond to the transitions from the ground state to the first (second) excited state. The error bars (smaller than the symbol sizes) are standard errors of fittings to derive EPR peak positions. The solid lines are the fitting curve calculated from the spin Hamiltonian. Inset: energy-level structure of the NV center in diamond. c EPR spectrum in several magnetic fields. The higher frequency peak in the 5.8-mT curve (purple) has a smaller frequency shift due to the limitation of the dynamic range of the spectrometer