The study of an ensemble of phase–coherent atoms has recently lead to interesting theoretical innovations and experimental demonstrations of counter–intuitive effects such as electromagnetically induced transparency (EIT), lasing without inversion (LWI), enhancement of index of refraction and ultra–large nonlinear susceptibility. In the present notes, we report a couple of new effects along these lines. In fact, it would be fair to call them surprises even in the repertoire of ‘counter–intuitive effects’, i.e. ‘counter–counter–intuitive effects’. Specifically, we will show that the LWI concept, which is based on quantum coherence, has a surprising counterpart in the classical physics of free electron laser operation. In other current work, we find that it is possible to ‘lock’ atoms in an excited state via atomic coherence. It is, by now, not surprising that such a phase–coherent ensemble can show holes or dark lines in the emission spectrum. It is surprising that we can lock atoms in an excited (normally decaying) state via atomic coherence and interference. Such a phase–coherent collection of atoms, i.e. ‘phaseonium’, is indeed a novel new state of matter.
The study of an ensemble of phase–coherent atoms has recently lead to interesting theoretical innovations and experimental demonstrations of counter–intuitive effects such as electromagnetically induced transparency (EIT), lasing without inversion (LWI), enhancement of index of refraction and ultra–large nonlinear susceptibility. In the present notes, we report a couple of new effects along these lines. In fact, it would be fair to call them surprises even in the repertoire of ‘counter–intuitive effects’, i.e. ‘counter–counter–intuitive effects’. Specifically, we will show that the LWI concept, which is based on quantum coherence, has a surprising counterpart in the classical physics of free electron laser operation. In other current work, we find that it is possible to ‘lock’ atoms in an excited state via atomic coherence. It is, by now, not surprising that such a phase–coherent ensemble can show holes or dark lines in the emission spectrum. It is surprising that we can lock atoms in an excited (normally decaying) state via atomic coherence and interference. Such a phase–coherent collection of atoms, i.e. ‘phaseonium’, is indeed a novel new state of matter.
Quantum coherence and correlations in atomic and radiation physics have led to many interesting and unexpected consequences. For example, an atomic ensemble prepared in a coherent superposition of states yields the Hanle effect, quantum beats, photon echo, self-induced transparency, and coherent Raman beats. In fact, in Section 1.4, we saw that the quantum beat effect provides one of the most compelling reasons for quantizing the radiation field. A further interesting consequence of preparing an atomic system in a coherent superposition of states is that, under certain conditions, it is possible for atomic coherence to cancel absorption. Such atomic states are called trapping states †. The observation of nonabsorbing resonances via atomic coherence and interference impacts on the concepts of lasing without inversion (LWI),‡ enhancement of the index of refraction accompanied by vanishing absorption, and electromagnetically induced transparency. In lasing without inversion, the essential idea is the absorption cancellation by atomic coherence and interference. This phenomenon is also the essence of electromagnetically induced transparency. Usually this is accomplished in three-level atomic systems in which there are two coherent routes for absorption that can destructively interfere, thus leading to the cancellation of absorption. A small population in the excited state can thus lead to net gain. A related phenomenon is that of resonantly enhanced refractive index without absorption in an ensemble of phase-coherent atoms ( phaseonium ). In a phaseonium gas with no population in the excited level, the absorption cancellation always coincides with vanishing refractivity.
Summary form only given. Intensive theoretical work, carried out in the past decade, has indicated that optical properties of matter can be dramatically affected by atomic coherence and interference effects. In particular, atomic coherence and interference was shown to lead to such intriguing phenomena as electromagnetically induced transparency, lasing without population inversion (LWI), and resonant enhancement of refractive index accompanied by vanishing absorption. In the present paper we report the first experimental observation of LWI oscillation and the first demonstration of index enhancement.
Recently, there has been increasing interest on the idea of controlling optical responses using atomic-coherence or quantum-interferences. Typical example may be the concept of electomagnetically-induced transparency (EIT) [1]. The key idea of EIT is to apply a strong-coupling field between a metastable and upper states to dress them equivalently as two states decaying to a common level or continuum, and to induce the transparency at the split-center of Autler-Townes components. This idea of EIT may open new possibilities to various coherent processes, such as lasing without population inversion, enhanced refractive index without absorption], slow group velocity without absorption, enhanced nonlinearity without absorption, etc.
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.
Optical response of an active medium can substantially be modified when coherent superpositions of states are excited, that is, when systems display quantum coherence and interference. This has led to fascinating applications in atomic and molecular systems. Examples include coherent population trapping, lasing without inversion, electromagnetically induced transparency, cooperative spontaneous emission, and quantum entanglement. \n \nWe study quantum coherence effects in several quantum optical systems and find interesting applications. We show that quantum coherence can lead to transient Raman lasing and lasing without inversion in short wavelength spectral regions--extreme ultraviolet and x-ray--without the requirement of incoherent pumping. For example, we demonstrate transient Raman lasing at 58.4 nm in Helium atom and transient lasing without inversion at 6.1 nm in Helium-like Boron (triply-ionized Boron). We also investigate dynamical properties of a collective superradiant state prepared by absorption of a single photon when the size of the sample is larger than the radiation wavelength. We show that for large number of atoms such a state, to a good approximation, decays exponentially with a rate proportional to the number of atoms. We also find that the collective frequency shift resulting from repeated emission and reabsorption of short-lived virtual photons is proportional to the number of species in the sample. Furthermore, we examine how a position-dependent excitation phase affects the evolution of entanglement between two dipole-coupled qubits. It turns out that the coherence induced by position-dependent excitation phase slows down the otherwise fast decay of the two-qubit entanglement. We also show that it is possible to entangle two spatially separated and uncoupled qubits via interaction with correlated photons in a cavity quantum electrodynamics setup. Finally, we analyze how quantum coherence can be used to generate continuous-variable entanglement in quantum-beat lasers in steady state and propose possible implementation in quantum lithography.
It is well known that the optical properties of multi-level atomic and molecular system can be controlled and manipulated efficiently using quantum coherence and interference, which has led to many new effects in quantum optics for e.g. lasing action without population inversion, ultraslow light, high resolution nonlinear spectroscopy etc. Recent experimental and theoretical studies have also provided support for the hypothesis that biological systems uses quantum coherence. Nearly perfect excitation energy transfer in photosynthesis is an excellent example of this.\n\nIn this dissertation we studied quantum coherence and interference effects in the transient and the continuous-wave regimes. This study led to (i) the first experimental demonstration of carrier-envelope phase effects on bound-bound atomic excitation in multi-cycle regime (~15 cycles), (ii) a unique possibility for standoff detection of trace gases using their rotational and vibrational spectroscopic signals and from herein called Coherent Raman Umklappscattering, (iii) several possibilities for frequency up-conversion and generation of short-wavelength radiation using quantum coherence (iv) the measurement of spontaneous emission noise intensity in Yoked-superfluorescence scheme.\n\nApplications of the obtained results are development of XUV (X-Ray) lasers, con- trolled superfluorescent (superradiant) emission, carrier-envelope phase effects, coherent Raman scattering in the backward direction, enhancement of efficiency for generating radiation in XUV and X-Ray regime using quantum coherence with and without population inversion and to extend XUV and X-Ray lasing to ~4.023 nm in Helium-like carbon.