State tomography is one of the essential tools in quantum science to analyze the quantum and classical nature of elementary excitations in solids. Elementary excitations in magnetic materials include magnons (spin waves) and their scattering processes determine various magnetic properties. Recently, magnon states characterized by unconventional fluctuations, such as squeezed states, mixed states, and entanglement between magnons, have been theoretically predicted. Thus, its experimental approach has been awaited with a view to its application to quantum and nonclassical computation using magnetic materials.
Here we have demonstrated a state tomography for magnetization dynamics that enables us to experimentally obtain the Wigner function, a probability distribution function that represents the fluctuation distribution of magnetization dynamics. By utilizing recently developed spin-current generation and detection techniques, we realized the observation of magnon fluctuations and the reconstruction of the Wigner function. With this method, we demonstrated that the coherence of precessional motion, which is conventionally thought to be lost in about 100 nanoseconds, is retained for microseconds. In the parametric excitation, two degrees of freedom, 0 and π, remain in the oscillation phase, and it is expected to be possible to generate various magnon states.