From spin ladders to the 2D O(3) model at non-zero density Original Research Article

The numerical simulation of various field theories at non-zero chemical potential suffers from severe complex action problems. In particular, QCD at non-zero quark density can presently not be simulated for that reason. A similar complex action problem arises in the 2D O(3) model—a toy model for QCD. Here we construct the 2D O(3) model at non-zero density via dimensional reduction of an antiferromagnetic quantum spin ladder in a magnetic field. The complex action problem of the 2D O(3) model manifests itself as a sign problem of the ladder system. This sign problem is solved completely with a meron cluster algorithm.