The breakdown of Born-Oppenheimer approximation

In dealing with electrons attached to nuclei, the Born-Oppenheimer approximation is often invoked, in which the motions of the nuclei are treated adiabatically relative to that of electrons. However, it will breakdown if the nuclei is light and the electron-nuclei coupling is very strong. In this case, the evolution of such electron-nuclei systems become coherent and entangled. A direct experimental demonstration of this breakdown was recently attained [Nature Physics, 1802(2010)].