  We derive Thouless-Anderson-Palmer (TAP) equations for quantum disordered
systems. We apply them to the study of the paramagnetic and glassy phases in
the quantum version of the spherical p spin-glass model. We generalize several
useful quantities (complexity, threshold level, etc.) and various ideas
(configurational entropy crisis, etc), that have been developed within the
classical TAP approach, to quantum systems. The analysis of the quantum TAP
equations allows us to show that the phase diagram (temperature-quantum
parameter) of the p spin-glass model should be generic. In particular, we argue
that a crossover from a second order thermodynamic transition close to the
classical critical point to a first order thermodynamic transition close to the
quantum critical point is to be expected in a large class of systems.
