Meron-cluster solution of fermion sign problems

We present a general strategy to solve the notorious fermion sign problem u sing cluster algorithms. The method applies to various systems in the Hubba rd model family as well as to relativistic fermions. Here it is illustrated for nonrelativistic lattice fermions. A configuration of fermion world lin es is decomposed into clusters that contribute independently to the fermion permutation sign. A cluster whose flip changes the sign is referred to as a meron. Configurations containing meron clusters contribute 0 to the path integral, while all other configurations contribute 1. The cluster represen tation describes the partition function as a gas of clusters in the zero-me ron sector.