Coherent control of magnon–polaritons using an exceptional point

In a non-Hermitian system, the amplitude of resonant oscillations can either grow or decay in time, corresponding to a mode with either gain or loss. When two coupled modes have a specific gain–loss imbalance, an exceptional point emerges at which both eigenfrequencies and eigenmodes of the system coalesce. Exceptional points have qualitative effects on the dynamics of systems due to their topological properties, and have been used to control systems including optical microcavities, the lasing of a parity–time-symmetric waveguide and terahertz pulse generation. A challenging open problem is the fully deterministic and direct manipulation of the systems’ loss and gain on timescales relevant to the coherent control of excitations. Here we demonstrate the rapid manipulation of the complex frequency of magnon–polaritons on durations much shorter than their decay rate, allowing us to exploit non-Hermitian physics for coherent control. By dynamically encircling an exceptional point, we demonstrate population transfer between coupled magnon–polariton modes. We then drive the system directly through an exceptional point, and demonstrate that this allows the coupled system to be prepared in an equal superposition of eigenmodes. These findings establish a highly controllable hybrid platform for exploring the rich dynamical properties of non-Hermitian systems. Deterministic control of the gain–loss balance in non-Hermitian systems remains challenging. A magnonic hybrid platform is now shown to enable this and, hence, coherently control excitations by leveraging an exceptional point.