  Quantum state diffusion shows how stochastic interaction with the environment
may cause localisation of the wave-function, and thereby demonstrates that
quantum mechanics need not invoke a separate axiom of measurement to explain
the emergence of the classical world. It has not been clear whether quantum
state diffusion requires some new physics. We set up an explicit numerical
calculation of the evolution of the wave-function of a two-state system under
interaction using only the physics explicitly contained in quantum mechanics
without an axiom of measurement. The wave-function does indeed localise, as
proposed by quantum state diffusion, on eigenstates of the perturbation. The
mechanism appears to be the superposition of histories evolving under different
Hamiltonians.
