A Practical Guide to using Pauli Path Simulators for Utility-Scale Quantum Experiments

Abstract

In this this paper we present an inexpensive protocol to perform runtime and memory estimation for large-scale experiments with Pauli Path simulators (PPS). Additionally, we propose a conceptually simple solution for studying whether PPS can be used as a scientific discovery tool, rather than reproducing existing answers. We start by analyzing the dynamics of the Pauli coefficients tracked in the Heisenberg picture. In addition to surprisingly generic convergence features of the Pauli coefficient distributions, we find certain regularities that allow for extrapolation of memory and runtime requirements for smaller and smaller coefficient truncation parameter $\delta$. We then introduce a framework for understanding convergence in the absence of rigorous error guarantees on PPS. Combined with runtime analysis, we propose bifurcating quantum simulation problems broadly into two classes, based on whether there is apparent convergence of expectation values as a function of $\delta$. This serves as a way for practitioners to understand where their problem falls on the frontier of classical simulability. In the case without apparent convergence, PPS may still serve useful as a Monte Carlo-like estimate. Applied to IBM's utility-scale experiments, we show parameter regimes where both behaviors are realized. Some of our key findings challenge conventional intuition: reducing $\delta$ does not always improve accuracy, and deeper quantum circuits may actually be easier to simulate than shallower ones. The BlueQubit SDK implementing these methods has been released publicly, offering researchers a comprehensive toolkit for evaluating this frontier classical simulation approach. These results establish practical guidelines for when PPS can serve as a reliable verification tool versus when it should be used as a complementary estimate alongside quantum experiments.