@article {1234, title = {Complexity of the XY antiferromagnet at fixed magnetization}, journal = {Quantum Information and Computation}, volume = {16}, year = {2016}, month = {2016/01/01}, pages = {1-18}, abstract = { We prove that approximating the ground energy of the antiferromagnetic XY model on a simple graph at fixed magnetization (given as part of the instance specification) is QMA-complete. To show this, we strengthen a previous result by establishing QMA-completeness for approximating the ground energy of the Bose-Hubbard model on simple graphs. Using a connection between the XY and Bose-Hubbard models that we exploited in previous work, this establishes QMA-completeness of the XY model. }, url = {http://arxiv.org/abs/1503.07083v1}, author = {Andrew M. Childs and David Gosset and Zak Webb} }