One may add that in the early days of quantum theory there was a tendency among the practitioners to think that quantum systems must always be in states of well-defined energy, and this led to various conundrums. Nowadays states which are superpositions of the ground state and an excited state of an atom are produced routinely in quantum optics experiments, and thus analogs of \(\EC_c\) can be quite relevant for a quantum description of what goes on in the laboratory.