  We show that cluster algorithms for quantum models have a meaning independent
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 equivalent. We also
provide numerical evidence confirming the analytic arguments. Similar
techniques are applicable to other models. In particular, in the recently
constructed quantum link models, the new technique allows us to construct
improved estimators for Wilson loops and may lead to a very precise
determination of the glueball spectrum.

Statistical Mechanics;High Energy Physics - Lattice