When an atom, in a linear superposition of states, absorbs or emits light, interference can occur between two different absorption or emission amplitudes, giving rise to observable physical effects, such as level crossing resonances, quantum beats, and dark states. Early experimental demonstrations of effects of this kind, using an optical detection of the atomic internal state, are briefly reviewed. In fact, such interference effects are still playing a fundamental role in modern developments, such as laser cooling and trapping. We present in this paper a few examples of such applications, including the mechanical Hanle effect, velocity selective resonances for an atom moving in a standing wave and subrecoil laser cooling by Velocity Selective Coherent Population Trapping (VSCPT).
In this paper, we theoretically demonstrate that electromagnetic transparency can be induced by optical magnetism in a metamaterial, which is composed of metamolecules. Each metamolecule consists of a metallic split-ring resonator, as one bright meta-atom (which is optically magnetic), and also a cut-wire pair, as one dark meta-atom (which is optically nonmagnetic). It is found that magnetic resonances occur at optical frequencies due to the local magnetic interaction between ``bright'' meta-atoms and ``dark'' meta-atoms; thereafter, a transparency window emerges upon the original absorption background. The phenomenon is similar to the electromagnetically induced transparency (EIT) in atomic three-level systems, and a microscopic picture is given to compare it with the EIT. Furthermore, low loss and slow light in this metamaterial have also been verified. The investigations may achieve potential applications on integrated optical circuits.
Artificial atom-like systems are a promising candidate for next generation quantum processing. Among them, dark excitons exhibit one of the longest lifetimes at high temperatures. Here, we demonstrate coherent control of dark excitonic states in metal-organic chalcogenolates (MOChas) by using an ultrafast pulse shaper at room temperature. These dark exciton states are optically accessed via two-photon absorption and directly read out with a four-wave mixing process. The system is described by a non-perturbative, two-photon Hamiltonian based on well-known atomic physics and applied to a three level system comprised of two dark excitons. Empirical and theoretical state specific optical access is shown via a simple optical pulse shape. The developed Hamiltonian-based description is a first step towards a quantum processing platform using three-level systems and two photon transitions, one example being dark excitons in the MOCha silver benzeneselenolate (mithrene). Simple conditions for gate operations are laid out and described.