Entanglement of electromagnetic fields

Given a quantum state of the electromagnetic field, one is, in principle, free to redefine the field modes. We show here how the amount of entanglement in a given state depends on redefinitions of the modes, and calculate the minimum and maximum entanglement over all such redefinitions for several examples. 

Redefinitions can also be interpreted as transformations that one can apply actively, for example, in order to create nonlocal entanglement.