Pauli Propagation: Simulating Quantum Spin Dynamics via Operator Complexity

Abstract: Simulating real-time quantum dynamics in interacting spin systems is a fundamental challenge, where exact diagonalization suffers from exponential Hilbert-space growth and tensor-network methods face entanglement barriers. In this work, we introduce a scalable Pauli propagation approach that evolves local observables directly in the Heisenberg picture. Theoretically, we derive a priori error bounds governed by the Operator Stabilizer R\'enyi entropy (OSE) $\mathcal{S}^\alpha(O)$, which explicitly links the truncation accuracy to operator complexity and prescribes a suitable Top-$K$ truncation strategy. For the 1D Heisenberg model with $J_z = 0$, we prove the number of non-zero Pauli coefficients scales quadratically in Trotter steps, establishing the compressibility of Heisenberg-evolved operators. Numerically, we validate the framework on XXZ Heisenberg chain benchmarks, showing high accuracy with small $K$ in free regimes ($J_z = 0$) and competitive performance against tensor-network methods (e.g., TDVP) in interacting cases ($J_z = 0.5$). These results establish an observable-centric simulator whose cost is governed by operator complexity rather than entanglement, offering a practical alternative for studying non-equilibrium dynamics in quantum many-body systems.