Intrinsic entanglement of photons
Multipole radiation is treated both classically and also quantum mechanically. Dipole atom as a source of radiation is investigated within the Jaynes-Cummings model. Polarization properties of quantum multipole radiation are given. It is shown that multipole photons have all three components of polarization but we can perform a local transformation of radiation frame such that the new z− axis corresponding to linear polarization becomes parallel to the Poynting vector. It is shown that the spin angular momentum and orbital angular momentum have the same operator structure, and in the far zone, they contribute equally to the total angular momentum. Hence in this regime, these two contributions are indistinguishable and they may differ from each other only by spatial dependence in the very vicinity of the source. Another aspect of the behavior in the far zone is that the longitudinal polarization of multipole photons vanish. A variational approach to entanglement which is introduced recently based on analysis of dynamic symmetry of systems and quantum uncertainties, accompanying the measurement of mean value of basic observables is applied to investigate the intrinsic entanglement of electric dipole photons. The basic observables are defined in terms of an orthogonal basis of Lie Algebra, corresponding to the dynamic symmetry group of the system of interest. It is shown that electric dipole photons can carry entanglement with respect to its intrinsic degrees of freedom, namely the spin angular momentum and orbital angular momentum, each of which may be considered as a qubit.