Hello! Assuming we can bring 2 energy levels very close to each other (e.g. by applying a magnetic field), what is the practical limit (in terms of lab equipment) on the smallest energy difference that we can measure? And what is the relative error on it, that can be obtained? For example if the 2 levels are 100 Hz apart, can I measure that difference in practice (e.g. by doing some sort of Rabi measurement with an RF generator)?