On Quantum Inequalities
  Building on the "quantum inequalities" introduced by Ford, I argue that the negative local energies encountered in quantum field theory can only be observed by detectors with positive energies at least as great in magnitude. This means that operationally the total energy density must be non-negative. Like reasoning shows that, in a similar operational sense, the dominant energy condition must hold: any timelike component of the four-momentum density is positive. 
Adam D. Helfer