Figure 2. Sampling the reduced density matrix (RDM) in path-integral quantum Monte Carlo (QMC). Panel (a) shows the partition function in the space time manifold appearing in usual QMC simulations. Panel (b) shows how the RDM is sampled in such QMC simulations, where one imposes open boundary conditions in time for the skeletal sites in subregion A. The lower panel in (b) demonstrates the spatial partition of A and B in such a setting.

Well, duh!