Showing papers on "Atomic coherence published in 1997"

Electromagnetically Induced Transparency

Abstract: Electromagnetically induced transparency is a technique for eliminating the effect of a medium on a propagating beam of electromagnetic radiation EIT may also be used, but under more limited conditions, to eliminate optical self‐focusing and defocusing and to improve the transmission of laser beams through inhomogeneous refracting gases and metal vapors, as figure 1 illustrates The technique may be used to create large populations of coherently driven uniformly phased atoms, thereby making possible new types of optoelectronic devices

Ion-trap laser

Abstract: Laser operation with a single trapped Ca+ ion in a high-finesse optical cavity is predicted at a wavelength of 866 nm. Different from a conventional laser, the formation of two thresholds can be observed. For both small and large pump strengths the field is quasi-thermal, in an intermediate region laser light with Poissonian and even sub-Poissonian statistics is produced. Atomic coherence effects play an important role. The multi-level system is treated by generalizing the pump-operator approach (Europhys. Lett., 33 (1996) 515).

Light amplification mechanisms in a coherently coupled atomic system

Abstract: We analyze transient properties of light amplification in a coherently coupled three-level \ensuremath{\Lambda}-type system and study the system evolution from the transient regime into the steady state. From the time evolution of the atomic coherence and population distribution, we discuss the transient light amplification mechanism with and without population inversion. We also solve the density-matrix equations in the steady state and derive the conditions for the existence of specific light amplification mechanisms, such as light amplification without inversion in any standard state basis, light amplification by stimulated Raman scattering, and light amplification from population inversion in the dressed states.

Atomic coherence effects in four-level systems: Doppler-free absorption within an electromagnetically-induced-transparency window

Abstract: We report here an effect in a four-level ladderlike system, which is in contrast to the usual quantum interference effects such as electromagnetically induced transperency (EIT) or coherent population trapping: we predict the occurrence of a narrow absorption peak within the EIT window when an EIT atomic system interacts with an additional driving rf field. The Doppler-free-central absorption appears when the three-photon resonance condition is satisfied. In the limit of the rf field strength ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{rf}}$\ensuremath{\rightarrow}0, the usual EIT profile is recovered.

Characterizing the coherence of Bose-Einstein condensates and atom lasers

Abstract: For a dilute, interacting Bose gas of magnetically-trapped atoms at temperatures below the critical temperature T0 for Bose-Einstein condensation, we determine the second-order coherence function g(2)(r1, r2) within the framework of a finite-temperature quantum field theory. We show that, because of the different spatial distributions of condensate and thermal atoms in the trap, g(2)(r1, r2) does not depend on |r1 - r2| alone. This means that the experimental determinations of g(2) reported to date give only its spatial average. Such an average may underestimate the degree of coherence attainable in an atom laser by judicious engineering of the output coupler.

Lasing without inversion: problems and prospects

Abstract: We review the recent progress both in the theory and in experiments on lasing without inversion and discuss the potential application of this phenomenon for the generation of coherent gamma radiation.

Generation of higher-order atomic dipole squeezing in high-Q micromaser cavity

Abstract: In this paper, we have introduced the concept of higher-order atomic dipole squeezing (HOADS) and have, as an example, applied it to a quantum optical model known as Jaynes-Cummings model (JCM) which could generate second-order atomic dipole squeezing (SOADS). We have shown that the JCM coulc also generate HOADS when it is restricted to the following initial condition: the field in a thermal state and the atom in an appropriate coherent state. We have studied in detail the influence of initial atomic coherence or thermal photons on HOADS, the important connection of HOADS with SOADS and second-order field squeezing (SOFS), and the possibility of realization of HOADS in micromaser experiments with Rydberg atoms in a high-Q cavity.

Quantum optics: Lasing without inversion and other effects of atomic coherence and interference

Abstract: 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.

Counter-counter-intuitive quantum coherence effects

Abstract: 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.

Atomic coherence and field-dependent relaxation in strongly driven three-level atoms

Abstract: We show that strong coherent field drastically modifies the spontaneous decay of three-level atoms when one dynamic Stark sublevel crosses a neighbouring atomic level.

Atomic coherence induced by two-photon process——transition from lasing with inversion to lasing without inversion

Abstract: In this paper, the pumping mechanism in the closed Ξ-type three-level system can be simplified using the coherence induced by the two-photon process. The system exhibits either lasing with inversion or lasing without inversion so long as there exists the pumping field. The system exhibits the transition from lasing with inverson to lasing without inversion under certain conditions. Near the transition lasing without inversion may be as efficient as lasing with inversion.

Quantum theory of nondegenerate multiwave mixing:effect of pump phase fluctuations

Abstract: We extend the quantum theory of nondegenerate four-wave mixing by including the effects of the finite bandwidth of the driving-pump field. The interaction of a beam of two-level atoms with the two opposite driving-pump fields that have finite bandwidth inside a bimodal cavity is considered. The master equation for the cavity-field modes, averaged over the stochastic process, is derived. We use our theory to study the effects of phase fluctuations, associated with the driving-pump field, on the generation of two-mode squeezing inside the cavity. The steady-state squeezing is achieved with the same driving-pump field as a local oscillator in the balanced homodyne-detection system. Our results show that, in spite of instantaneous phase locking between the cavity field and the local oscillator, the time-delay effects associated with the exponential decay of atomic coherence relate the steady-state squeezing to the diffusion constant of the driving-pump field.

Control of optical properties of materials by atomic coherence and quantum interference effects

Abstract: We investigate the spontaneous emission spectrum of a three-level atom coherently driven by a microwave field. The microwave excitation establishes a coherent link between the bottom two levels, and as a consequence induces the quantum interference of previously independent decay pathways. As the decay pathways differ from one another by an odd number of stimulated emission processes, the spontaneous emission spectrum acquires a dependence on the phase of the excitation field, a feature that is absent in other resonance fluorescence phenomena.