Generalized Reduced-Density-Matrix Quantum Monte Carlo Gives Access to More

In quantum Monte Carlo (QMC), what can be measured efficiently is largely determined by what is sampled. When the sampled object is the partition function, a broad class of observables, including general off-diagonal operators, is typically unavailable as direct estimators. In this article, we introduce a paradigm shift by replacing the partition function with a generalized reduced density matrix (GRDM) as the simulated object. This reformulation removes the measurement bottleneck at its source and extends the dimensional-reduction advantage of reduced descriptions from static quantities to dynamical observables, thereby enabling much richer information extraction. As substantial demonstrations, the framework allows the directed-loop algorithm to measure both equal-time and imaginary-time off-diagonal observables, with the latter giving direct access to dynamical spectra. It also enables measurements of Rényi-1 correlators that diagnose strong-to-weak symmetry breaking in mixed states. This work establishes a unified framework for holographic characterization within QMC.

Source: arXiv:2603.10948v1 - http://arxiv.org/abs/2603.10948v1 PDF: https://arxiv.org/pdf/2603.10948v1 Original Link: http://arxiv.org/abs/2603.10948v1