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arXiv:1606.00114

Beyond Quantum interference and Optical pumping: invoking a Closed-loop phase

A. Kani, Harshawardhan Wanare

Published Jun 1, 2016• 0 citations
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Abstract

Atomic coherence effects arising from coherent light-atom interaction are conventionally known to be governed by quantum interference and optical pumping mechanisms. However, anisotropic nonlinear response driven by optical field involves another fundamental effect arising from closed-loop multiphoton transitions. This closed-loop phase dictates the tensorial structure of the nonlinear susceptibility as it governs the principal coordinate system in determining, whether the light field will either compete or cooperate with the external magnetic field stimulus. Such a treatment provides deeper understanding of all magneto-optical anisotropic response. The magneto-optical response in all atomic systems is classified using closed-loop phase. The role of quantum interference in obtaining electromagnetically induced transparency or electromagnetically induced absorption in multi-level systems is identified.

DOI
10.48550/arxiv.1606.00114

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Atomic coherence effects arising from coherent light-atom interaction are conventionally known to be governed by quantum interference and optical pumping mechanisms. However, anisotropic nonlinear response driven by optical field involves another fundamental effect arising from closed-loop multiphoton transitions. This closed-loop phase dictates the tensorial structure of the nonlinear susceptibility as it governs the principal coordinate system in determining, whether the light field will either compete or cooperate with the external magnetic field stimulus. Such a treatment provides deeper understanding of all magneto-optical anisotropic response. The magneto-optical response in all atomic systems is classified using closed-loop phase. The role of quantum interference in obtaining electromagnetically induced transparency or electromagnetically induced absorption in multi-level systems is identified.
\n\nWeak coherent radiation interacting with multilevel atomic systems can excite a family of higher-order multiphoton transitions in contravention to conventional perturbative analysis. Monochromatic excitation leads to a strong anisotropic response arising from the interference of numerous underlying multiphoton pathways. The interference can be controlled through the polarization content of the input radiation. This anisotropic response is subnatural, quite unlike the anisotropy arising from spontaneous-emission–assisted optical pumping. Its sensitivity to external influences such as magnetic field suggests its utility in magnetometry. Specifically, we discover the presence of an elemental closed loop involving series of absorption and emission cycles that is central to the total response, and is universal to all atomic configurations. However, as not all atomic configurations support such anisotropy, we present a general classification of atomic transitions that exhibit anisotropic nonlinear response through quantum interference.\n
The appearance of quantum interference in the microscopic world is one of the deepest mysteries at the very root of quantum mechanics. When light interacts with atoms, it can induce transitions by way of distinct but indistinguishable pathways and yield unexpected and often counter-intuitive results. Electromagnetically Induced Transparency (EIT), Coherent Population Trapping (CPT), and Lasing Without Inversion (LWI) are modern examples of phenomena where the traditional rules that govern absorption and dispersion undergo major revisions. Spectacular consequences of this new state of affairs include the ability of a light beam to propagate through a normally absorbing medium with little or no absorption, or to undergo amplification even if the active medium is not prepared in a state of population inversion. Light signals have been shown to travel with strongly subluminal group velocities of only a handful of meters per second and, even, to propagate at speed greater than the ordinary speed of light, but without violation of causality or of the established rules that control the transfer of optical information through space. Three-level atoms can interact with two coherent electromagnetic fields according to three different systems, known as Cascade, Lambda and Vee systems. The propagation of a weak probe field can be deeply affected by the presence of a second stronger beam, the so-called coupling field. Transparency windows, splitting of emission and absorption lines and enormously enhanced dispersion have been documented in numerous experiments. In this thesis we explore the physical origin of the transparency induced in these systems by the simultaneous interplay of the coupling and probe fields. We focus our attention on the Cascade system and study its two configurations, Cascade-EIT and Cascade-AT. We develop a variety of complementary approaches for the description of these two configurations, some semiclassical and others fully quantum mechanical. We prove the existence of quantum interference in Electromagnetically Induced Transparency and also the surprising absence of interference in a closely related phenomenon, known as the Autler-Townes effect. Finally we complement the traditional machinery of theoretical quantum optics with techniques borrowed from quantum scattering theory, and offer what we believe is the most convincing physical evidence for the appearance, or for the absence, of quantum interference effects.
We have shown that quantum interference in a driven quasi-degenerate two-level atomic system can be controlled by an externally applied magnetic field. We demonstrate that the mechanism of optical control is based on quantum interference, which allows one to implement both electromagnetically induced transparency and electromagnetically induced absorption in one atomic system. Dispersion of such the medium allows one to control group velocity of propagation of light pulses be ultra-slow or superluminal via applied magnetic field.
Light scattered by a regular structure of atoms exhibits spatial interference signatures, similar to Young's classical double-slit experiment. The first-order interferences, however, are known to vanish for strong light intensities, where the incoherently fluctuating part of the emitted light dominates. Here, we show how to overcome these limitations to quantum interference in stronger laser fields, and how to recover the first-order interference in strong fields by a tailored electromagnetic vacuum with a suitable frequency dependence. We also discuss higher-order correlation functions of the scattered light, with applications, e.g., to lithography. In the second part, we study light propagation of a probe field pulse in closed-loop atomic systems. The closed interaction loop induces a sensitivity to the relative driving field phase, but in general prohibits a stationary steady state. In particular, the finite frequency width of the short probe pulse requires a time-dependent analysis beyond the so-called multiphoton resonance assumption. Using a Floquet decomposition, we identify the different contributions to the medium response, and demonstrate sub- and superluminal light propagation with small absorption or even gain, where a coupling field Rabi frequency allows to switch between sub- and superluminal light propagation.
Summary form only given. Linear and nonlinear atomic response can be manipulated by quantum interference in an atomic medium in which electromagnetically induced transparency (EIT) is created by a coherent coupling field. Recently Agarwal and Harshawardhan showed that two-photon absorption may be enhanced or suppressed by quantum interference in a ladder-type atomic system (Phys. Rev. Lett. vol. 77, p. 1039, 1996). We report an experimental demonstration of two-photon suppression by quantum interference in /sup 87/Rb atoms cooled and confined in a magneto-optical trap (MOT).