We study the complexity of approximating the partition function of
the q-state Potts model and the closely related Tutte polynomial
for complex values of the underlying parameters. Apart from the
classical connections with quantum computing and phase transitions
in statistical physics, recent work in approximate counting has
shown that the behaviour in the complex plane, and more precisely
the location of zeros, is strongly connected with the complexity of
the approximation problem, even for positive real-valued parameters.
Previous work in the complex plane by Goldberg and Guo focused on
q = 2, which corresponds to the case of the Ising model; for q > 2,
the behaviour in the complex plane is not as well understood and
most work applies only to the real-valued Tutte plane. Our main
result is a complete classification of the complexity of the
approximation problems for all non-real values of the parameters, by
establishing #P-hardness results that apply even when restricted to
planar graphs. Our techniques apply to all q $$\geq$$
≥
2 and further
complement/refine previous results both for the Ising model and the
Tutte plane, answering in particular a question raised by Bordewich,
Freedman, Lovász and Welsh in the context of quantum
computations.