  We consider the Dicke Hamiltonian, a simple quantum-optical model which
exhibits a zero-temperature quantum phase transition. We present numerical
results demonstrating that at this transition the system changes from being
quasi-integrable to quantum chaotic. By deriving an exact solution in the
thermodynamic limit we relate this phenomenon to a localisation-delocalisation
transition in which a macroscopic superposition is generated. We also describe
the classical analogues of this behaviour.
