  Monte Carlo simulations away from half-filling suffer from a sign problem
that can be reduced by deforming the contour of integration. Such a
transformation, which induces a Jacobian determinant in the Boltzmann weight,
can be implemented using neural networks. This additional determinant cost for
a generic neural network scales cubically with the volume, preventing
large-scale simulations. We implement a new architecture, based on
complex-valued affine coupling layers, which reduces this to linear scaling. We
demonstrate the efficacy of this method by successfully applying it to systems
of different size, the largest of which is intractable by other Monte Carlo
methods due to its severe sign problem.
