Fundamental Quantum Limit to Multiphoton Absorption Rate for   Monochromatic Light
  The local multiphoton absorption rate for an arbitrary quantum state of monochromatic light, taking into account the photon number, momentum, and polarization degrees of freedom, is shown to have an upper bound that can be reached by coherent fields. This surprising result rules out any quantum enhancement of the multiphoton absorption rate by momentum entanglement. 
Mankei Tsang (Massachusetts Institute of Technology)