  The dipole approximation is usually employed to describe light-matter
interactions under ordinary conditions. With the development of artificial
atomic systems, `giant atom' physics is possible, where the scale of atoms is
comparable to or even greater than the wavelength of the light they interact
with, and the dipole approximation is no longer valid. It reveals interesting
physics impossible in small atoms and may offer useful applications. Here, we
experimentally demonstrate the giant spin ensemble (GSE), where a ferromagnetic
spin ensemble interacts twice with the meandering waveguide, and the coupling
strength between them can be continuously tuned from finite (coupled) to zero
(decoupled) by varying the frequency. In the nested configuration, we
investigate the collective behavior of two GSEs and find extraordinary
phenomena that cannot be observed in conventional systems. Our experiment
offers a new platform for `giant atom' physics.

Quantum Physics