  Depending on the relative rates of coupling and dissipation, a light-matter
coupled system is either in the weak- or strong-coupling regime. Here, we
present a unique system where the coupling rate continuously increases with an
externally applied magnetic field while the dissipation rate remains constant,
allowing us to monitor a weak-to-strong coupling transition as a function of
magnetic field. We observed a Rabi splitting of a terahertz magnon mode in
yttrium orthoferrite above a threshold magnetic field of ~14 T. Based on a
microscopic theoretical model, we show that with increasing magnetic field the
magnons transition into magnon polaritons through an exceptional point, which
will open up new opportunities for in situ control of non-Hermitian systems.
