Shugo Yoshii is a postdoctoral researcher at Imperial College London in the UK. He received his B.S., M.Eng., and Ph.D. in Engineering from Kyoto University under the supervision of Professor Masashi Shiraishi. His research interests lie in magnonics in thin ferromagnetic films, particularly in magnon–photon polaritons excited within ferromagnetic layers only a few tens of nanometers thick. His recent results demonstrate that these polaritons enter the ultrastrong coupling regime, where non-negligible counter-rotating terms give rise to quantum entanglement between magnons and photons. His work lays the groundwork for exploring new frontiers at the intersection of quantum optics and spintronics.