Error reduction using covariance in quantum Monte Carlo simulations

A scheme utilizing covariance effects to reduce statistical errors of quantum Monte Carlo (QMC) calculations is presented. The same method can also significantly reduce certain types of systematical errors. The procedure can be carried out post-simulation. with data generated using standard QMC techniques Here it is demonstrated for one- and two-dimensional Heisenberg antiferromagnets. Statistical errors of spin-spin correlation functions can br reduced by up to orders of magnitude.