On the Emergence of a Polychromatic Photon from a Single Atom

Recent attempts to establish a space‐time description of single‐photon emission from single atoms (or molecules) are reviewed. In the wake of historical remarks on the so‐called Lorenz zone of molecular influence, characteristics of the power‐law confined birth domain of the photon are discussed within the framework of a field‐unquantized formalism. In contrast to the common believe it is shown that the birth domain can be (and is) exponentially localized if the photon emerges from a pure spin transition. As a paradigm of exponential localization we discuss the emergence of a radio‐frequency photon from the hydrogen hyperfine structure. Starting from the first‐quantized description of the energy wave function of a polychromatic photon in vacuo, we introduce a propagator description of the photon embryo, and argue that the single‐photon tunnelling process is a fingerprint of our inability to localize photons in space‐time. With this understanding optical tunnelling appears as an indispensable part of all single photon‐atom scattering processes. In the final part of the review we briefly describe how the formalism can be upgraded from single‐photon wave mechanics to a field‐quantized description based on wave packet (wave train) electrodynamics. In this new formalism, in which the polychromatic photon embryo is included as an integral part of the polychromatic field quanta, the electrons of the atom (or molecule) move not only in the self‐consistent Coulomb potential but also in the transverse potential created by the particles themselves. A rigorous relativistic quantum electrodynamic theory of wave train photon emission from single atoms emerges in the horizon.