Virtual photons, dipole fields and energy transfer: a quantum electrodynamical approach

Dipole emission mechanisms for energy transfer operate in many important areas of photophysics. A straightforward analysis based on quantum electrodynamics not only reveals the entanglement of mechanisms usually regarded as 'radiative' and 'radiationless'; it also gives significant physical insights into a host of topics in electromagnetism. These include: the designation of real and virtual photons; propagating and non-propagating character in electromagnetic fields; near-zone and wave-zone effects; transverse and longitudinal character; the effects of retardation; the relation between couplings of static and transition dipoles, and manifestations of quantum uncertainty. A simple extension of the theory to accommodate magnetic dipole as well as electric dipole transitions furthermore reveals key differences between the range dependences of the magnetic and electric fields produced by dipolar emission. With important technological applications, this lesson in advanced physics showpieces the interplay of principles associated with quantum mechanics, electromagnetism and photophysics.