  In sharp contrast to classical proof complexity we are currently short of
lower bound techniques for QBF proof systems. In this paper we establish the
feasible interpolation technique for all resolution-based QBF systems, whether
modelling CDCL or expansion-based solving. This both provides the first general
lower bound method for QBF proof systems as well as largely extends the scope
of classical feasible interpolation. We apply our technique to obtain new
exponential lower bounds to all resolution-based QBF systems for a new class of
QBF formulas based on the clique problem. Finally, we show how feasible
interpolation relates to the recently established lower bound method based on
strategy extraction.
