  We apply a meron cluster algorithm to the XY spin chain, which describes a
quantum rotor. This is a multi-cluster simulation supplemented by an improved
estimator, which deals with objects of half-integer topological charge. This
method is powerful enough to provide precise results for the model with a
theta-term - it is therefore one of the rare examples, where a system with a
complex action can be solved numerically. In particular we measure the
correlation length, as well as the topological and magnetic susceptibility. We
discuss the algorithmic efficiency in view of the critical slowing down. Due to
the excellent performance that we observe, it is strongly motivated to work on
new applications of meron cluster algorithms in higher dimensions.
