Radiation from sources of any state of coherence

H. Baltes, Pierre Meystre, A. Quattropani

Iñigo Liberal, Íñigo Ederra, Yue Li

Classical antenna theory establishes that there are no coherent isotropic radiators. Thermal radiators can be isotropic, but they lack temporal coherence. Here, we demonstrate that quantum interference processes enable the design of isotropic and unpolarized single-photon sources that preserve temporal coherence properties. We illustrate how this effect can be realized with relatively simple multilevel emitters with degenerate ground states, and we discuss potential implementations based on atomic and solid-state single-photon sources.

Vladimir P Bykov

The quantum-mechanical state of laser radiation near the generation threshold is studied. It is shown that there is a second threshold in laser generation which lies above the first one. At the second threshold coherence appears in laser radiation. In a small region between the two thresholds the laser radiation is in an essentially quantum state, in which the average field strength is close to zero, but the average energy keeps a macroscopic value. Active atoms in superposition states are the sources of coherence in lasers. The laser active medium sufficiently enlarges the characteristic time of coherence.

H. Baltes

Keith R. Burnett

The classical theory of the interaction of radiation with matter developed by Einstein is part of the bag of tools of any scientist involved in radiation theory – the number of undergraduates who have been confused by ‘A’ and ‘B’ coefficients must be legion. Einstein's way of describing how photons are absorbed and emitted is so deeply ingrained in most scientists' understanding of radiation processes that they may forget that it is an approximate view of affairs. Classical broadband light sources – lamps – produce light whose influence on atoms can be described in terms of shunting populations between states. If one has a laser source, however, these simple pictures of population shunting may break down completely. It is essential when dealing with coherent sources to go back to the beginning and consider the dynamics of the radiation – matter interaction in detail.