No limit on the energy extracted from a Dirac quantum state
  When a Dirac quantum state interacts with an applied electric field the energy of the quantum state changes. It is generally assumed that there is a maximum limit on the amount of energy that can be extracted from a Dirac quantum state, due to its interaction with an electric field. In this article it is shown that this assumption is not correct and that, for a properly applied electric field, an arbitrarily large amount of energy can be extracted from the quantum state. 
Dan Solomon