  We use trapped atomic ions forming a hybrid Coulomb crystal, and exploit its
phonons to study an isolated quantum system composed of a single spin coupled
to an engineered bosonic environment. We increase the complexity of the system
by adding ions and controlling coherent couplings and, thereby, we observe the
emergence of thermalization: Time averages of spin observables approach
microcanonical averages while related fluctuations decay. Our platform features
precise control of system size, coupling strength, and isolation from the
external world to explore the dynamics of equilibration and thermalization.
