Paschen-Back effects and Rydberg-state diamagnetism in vapor-cell electromagnetically induced transparency

We report on a rubidium vapor-cell Rydberg electromagnetically induced transparency (EIT) experiment in a 0.7~T magnetic field where all involved levels are in the hyperfine Paschen-Back regime, and the Rydberg state exhibits a strong diamagnetic interaction with the magnetic field. Signals from both $^{85}\mathrm{Rb}$ and $^{87}\mathrm{Rb}$ are present in the EIT spectra. This feature of isotope-mixed Rb cells allows us to measure the strength of strong magnetic fields to within a $0.2$\% relative uncertainty. The measured spectra are in excellent agreement with the results of a Monte Carlo calculation. Line shifts and broadenings due to small inhomogeneities of the magnetic field are included in the model. The method can be extended to even higher fields.