Ensemble Optimization Techniques for Classical and Quantum Systems

593 We present a review of extended ensemble methods and ensemble optimization techniques. Extended ensemble methods, such as multicanonical sampling, broad histograms, or parallel tempering aim to accelerate the simulation of systems with large energy barriers, as they occur in the vicinity of first order phase transitions or in complex systems with rough energy landscapes, such as spin glasses or proteins. We present a recently developed feedback algorithm to iteratively achieve an optimal ensemble, with the fastest equi-libration and shortest autocorrelation times. In the second part we review time-discretization free world line representations for quantum systems, and show how any algorithm developed for classical systems, such as local updates, cluster updates or the extended and optimized ensemble methods can also be applied to quantum systems. An overview over the methods is followed by a selection of typical applications.