Green's functions from quantum cluster algorithms

We show that cluster algorithms for quantum models have a meaning independe nt of the basis chosen to construct them. Using this idea, we propose a new method for measuring with little effort a whole class of Green's functions , once a cluster algorithm for the partition function has been constructed. To explain the idea, we consider the quantum XY model and compute its two point Green's function in various ways, showing that all of them are equiva lent. We also provide numerical evidence confirming the analytic arguments. Similar techniques are applicable to other models. In particular, in the r ecently constructed quantum link models, the new technique allows us to con struct improved estimators for Wilson loops and may lead to a very precise determination of the glueball spectrum. (C) 1998 Published by Elsevier Scie nce B.V. All rights reserved.