  A damped and driven collective spin system is analyzed by using quantum state
diffusion. This approach allows for a mostly analytical treatment of the
investigated non-equilibrium quantum many body dynamics, which features a phase
transition in the thermodynamical limit. The exact results obtained in this
work, which are free of any finite size defects, provide a complete
understanding of the model. Moreover, the trajectory framework gives an
intuitive picture of the two phases occurring, revealing a spontaneously broken
symmetry and allowing for a qualitative and quantitative characterization of
the phases. We determine exact critical exponents, investigate finite size
scaling, and explain a remarkable non-algebraic behaviour at the transition in
terms of torus hopping.

Quantum Physics