Unconventional Thermalization of a Three-Wave-Mixing Model

Understanding the boundaries between quantum thermalization and localization in many-body systems remains a central frontier of condensed matter and quantum information science. In this work, we investigate the dynamics and spectral properties of a generic model with long-range three-body-interaction, namely, a system with non-local three-wave-mixing. This model has been realized recently with a microwave Fabry-Perot cavity terminated on one end by a superconducting qubit mirror. Utilizing exact diagonalization techniques, we uncover a striking paradox: the global energy level spacing statistics show integrability, even though all dynamic observables and inverse participation ratios of the eigenstates indicate ergodicity and delocalization. We show that this behavior is a hallmark of strong Hilbert space fragmentation driven by kinematic constraints rather than an explicit global symmetry. Inside these sectors, dynamics scramble rapidly, as evidenced by the out-of-time-ordered correlator (OTOC), while global transport is heavily bottlenecked, resulting in a logarithmic relaxation to equilibrium. This picture is further confirmed by fluctuations in eigenstate entanglement entropy at the same energy. Finally, we demonstrate that the late time OTOC average scales with system size, providing a distinct experimentally accessible signature of the underlying three-body kinetic bottlenecks.