  One of the most fascinating topics in current quantum physics are hybridised
systems, in which different quantum resonators are strongly coupled. Prominent
examples are circular resonators with high quality factors that allow the
coupling of optical whispering gallery modes to microwave cavities or magnon
resonances in optomagnonics. Whispering gallery modes play a special role in
this endeavour because of their high quality factor and strong localisation,
which ultimately increases the overlap of the wavefunctions of quantum
particles in hybridised systems. The hybridisation with magnons, the collective
quantum excitations of the electron spins in a magnetically ordered material,
is of particular interest because magnons can take over two functionalities:
due to their collective nature they are robust and can serve as a quantum
memory and, moreover, they can act as a wavelength converter between microwave
and THz photons. However, the observation of whispering gallery magnons has not
yet been achieved due to the lack of efficient excitation schemes for magnons
with large wave vectors in a circular geometry. To tackle this problem, we
studied nonlinear 3-magnon scattering as a means to generate whispering gallery
magnons. This Letter discusses the basics of this nonlinear mechanism in a
confined, circular geometry from experimental and theoretical point of view.
