  Quantum Phase Transition (QPT) is a phase transition between different
quantum states by adjusting some control parameters. Based on the Principle of
Hamilton Dynamics (PHD) and the Principle of Lagrangian Dynamics (PLD), a
general QPT model is established. Also, a definition of QPT is given. The
important point is that the QPT model is applied to Bose-Einstein Condensate
(BEC) and the corresponding bifurcation solutions (i.e., the quantum states)
are obtained by using steady state bifurcation theory.
