Quantum-mechanical theory of optical coherence - Laser-based precision spectroscopy and optical frequency comb techniques General introduction

General introduction Light provides the most pertinent example of the dual nature of quantum objects; its oscillatory properties served to verify the electromagnetic theory of Maxwell, and its lumpiness, the photons, signalled the dawn of modern quantum theory. The electromagnetic phenomena form an integral part of modern technology. They are at work in all electrical motors, and our communication devices utilize their oscillatory behaviour in essential ways. Our radio receivers and mobile phones are all based on the ability of the radiation to sustain well-defined frequency and phase properties. On the other hand, each device detecting the radiation must be based on the absorption of radiation energy into the material medium. This energy is known to occur in packets, which are now called photons. Absorption of a photon will cause the creation of an excitation which may be amplified and detected. Since the work of Einstein in 1905, we know that the absorption of a quantum of radiation gives rise to one and only one photoemission electron from a solid (Nobel Prize, 1921). Thus the detector counts photoelectrons and not photons, and our information about the behaviour of photons is always indirect. In the process of observation, the photon must be absorbed, and thus it is no longer available afterwards. The dualism between the two pictures may appear contradictory. They do, however, form the prime example of what is termed complementarity in quantum theory, namely the possibility to display either wave or particle properties; albeit they emerge in mutually exclusive limits. From a fundamental point of view, we need to reconcile the two descriptions. We must know how the seemingly smooth oscillatory behaviour of the radiation can manifest itself through the lumpy quantum nature of the field. We thus need both a macroscopic theory to account for the phase properties and a microscopic theory to account for the interaction between the photons and the material absorbing them. The former is given by Maxwell’s theory and the latter by quantum electrodynamics.