A comprehensive description of nuclear dynamics represents a central challenge in modern nuclear physics. It requires a simultaneous inclusion of two distinct physical mechanisms: the quantum fluctuations in the collective space and the dissipation between single-particle and collective degrees of freedom. However, state-of-the-art models typically include only one of these mechanisms, leading to serious limitations. In this talk, I will discuss a recently implemented model [1], based on the time-dependent density functional theory, that accounts for both mechanisms by mixing several time-dependent Hartree-Fock trajectories. The first application to atomic nuclei revealed the spontaneous emergence of collective multi-phonon excitations - high-energy nuclear modes observed in several experiments, yet notoriously hard to describe theoretically without a priori assuming their existence. The collective spectrum, found to be nearly harmonic, is in excellent agreement with the experimentally observed three quanta of the isoscalar giant quadrupole resonance in 40Ca. Further extensions of the model, both theoretical and computational, represent one of the most promising avenues for a comprehensive description of complex nuclear phenomena such as low-energy heavy-ion collisions or nuclear fission.