Quantum Coherence in a Maximally Hot Hubbard Chain

Abstract: We present a detailed study of the real-time dynamics and spectral properties of the one-dimensional fermionic Hubbard model at infinite temperature. Using tensor network simulations in Liouville space, we compute the single-particle Green's function and analyze its dynamics across a broad range of interaction strengths. To complement the time-domain approach, we develop a high-resolution Chebyshev expansion method within the density matrix formalism, enabling direct access to spectral functions in the frequency domain. In the non-interacting limit, we derive exact analytical expressions for the Green's function, providing a benchmark for our numerical methods. As interactions are introduced, we observe a transition in the spectral function from a sharp peak at the free dispersion to a broadened two-band structure associated with hole and doublon excitations. These features are well captured by a Hubbard-I mean-field approximation, even at intermediate coupling. At infinite interaction strength ($U = \infty$), we exploit a determinant representation of the Green's function to access both real-time and spectral properties. In this regime, the system retains a sharp, cosine-like momentum dispersion in frequency space, while the dynamics display nontrivial light-cone spreading with sub-ballistic scaling. Our results demonstrate that strong correlations and nontrivial quantum coherence can persist even at infinite temperature.