Showing papers on "Atomic coherence published in 2008"

Control of the Goos-Hänchen shift of a light beam via a coherent driving field

Abstract: We present a proposal to manipulate the Goos-H\"anchen shift of a light beam via a coherent control field, which is injected into a cavity configuration containing the two-level atomic medium. It is found that the lateral shifts of the reflected and transmitted probe beams can be easily controlled by adjusting the intensity and detuning of the control field. Using this scheme, the lateral shift at the fixed incident angle can be enhanced (positive or negative) under the suitable conditions on the control field, without changing the structure of the cavity.

Double resonance optical pumping effects in electromagnetically induced transparency

Abstract: We present the double resonance optical pumping (DROP) effect of ladder-type electromagnetically induced transparency (EIT) in the 5S1/2- 5P3/2-5D5/2 transition of 87Rb atoms. When many atoms of the ladder-type atomic system are simultaneously resonant with the two laser fields, the population of one ground state can be optically pumped into another ground state through intermediate states and excited states. In this paper, we reveal that most previous results for the ladder-type EIT include the DROP effect. When the probe laser is very weak and the coupling laser is strong, we can observe the double structure transmittance spectrum, a narrow spectrum due to the EIT and a broad spectrum due to the DROP, in the 5S1/2(F=2)- 5P3/2(F’=3)-5D5/2(F”=4) cycling transition.

Efficient energy transfer between four-wave-mixing and six-wave-mixing processes via atomic coherence

Abstract: We demonstrate efficient energy exchange during propagation between four-wave-mixing (FWM) and six-wave-mixing (SWM) signals generated in a four-level inverted-Y atomic system, which fall in the electromagnetically induced transparency window After an initial growth distance for both FWM and SWM fields in the atomic medium, these two nonlinear optical processes compete and exchange energy between them, and eventually reach their respective steady-state values at long interaction distance This energy exchange phenomenon can be explained by considering established atomic coherences among various atomic states and quantum interferences between three-photon and five-photon excitation pathways Understanding these high-order nonlinear optical processes and interplays between them can be very important for correlated FWM (or SWM) photon-pair generations and quantum information processing

Theory of thermal motion in electromagnetically induced transparency: Effects of diffusion, Doppler broadening, and Dicke and Ramsey narrowing

Abstract: We present a theoretical model for electromagnetically induced transparency (EIT) in vapor that incorporates atomic motion and velocity-changing collisions into the dynamics of the density-matrix distribution Within a unified formalism, we demonstrate various motional effects, known for EIT in vapor: Doppler broadening of the absorption spectrum; Dicke narrowing and time-of-flight broadening of the transmission window for a finite-sized probe; diffusion of atomic coherence during storage of light and diffusion of the light-matter excitation during slow-light propagation; and Ramsey narrowing of the spectrum for a probe and pump beams of finite size

Phase-locked laser system for use in atomic coherence experiments.

Abstract: We describe a phase-coherent laser system designed for use in experiments involving coherently prepared atomic media. We implement a simple technique based on a sample-and-hold circuit together with a reset of the integrating electronics that makes it possible to scan continuously the relative frequency between the lasers of over tens of gigahertz while keeping them phase locked. The system consists of three external-cavity diode lasers operating around 795 nm. A low-power laser serves as a frequency reference for two high-power lasers which are phased locked with an optical phase-locked loop. We measured the residual phase noise of the system to be less than 0.04 rad(2). In order to show the application of the system towards atomic coherence experiments, we used it to implement electromagnetically induced transparency in a rubidium vapor cell and obtained a reduction in the absorption coefficient of 92%.

Optical quantum memory with generalized time-reversible atom-light interactions

Abstract: We examine a quantum memory scheme based on controllable dephasing of atomic coherence of a non-resonant, inhomogeneously broadened Raman transition. We show that it generalizes the physical conditions for time-reversible interaction between light and atomic ensembles from weak to strong fields and from linear to non-linear interactions. We also develop a unified framework for different realizations exploiting either controlled reversible inhomogeneous broadening or atomic frequency combs, and discuss new aspects related to storage and manipulation of quantum states.

Continuous-variable entanglement in a two-mode four-level single-atom laser

Abstract: This paper investigate the generation and evolution of continuous-variable entanglement in a two-mode single-atom laser, where the atomic coherence is induced by two classical laser fields (pump and coupling field) driving the corresponding atom transitions. It is found that the intensity of the coupling field can influence effectively the entanglement period of the cavity field. More importantly, our numerical results also show that the intensity and period of entanglement between the two cavity modes as well as the total mean photon number of the cavity field can be increased synchronously by adjusting the corresponding frequency detuning.

Quantum memory with optically trapped atoms.

Abstract: We report the experimental demonstration of quantum memory for collective atomic states in a fardetuned optical dipole trap. Generation of the collective atomic state is heralded by the detection of a Raman scattered photon and accompanied by storage in the ensemble of atoms. The optical dipole trap provides confinement for the atoms during the quantum storage while retaining the atomic coherence. We probe the quantum storage by cross correlation of the photon pair arising from the Raman scattering and the retrieval of the atomic state stored in the memory. Nonclassical correlations are observed for storage times up to 60 � s.

State-selective imaging of cold atoms

Abstract: Atomic coherence phenomena are usually investigated using single beam techniques without spatial resolution. Here we demonstrate state-selective imaging of cold $^{85}\text{R}\text{b}$ atoms in a three-level ladder system, where the atomic refractive index is sensitive to the quantum coherence state of the atoms. We use a phase-sensitive diffraction contrast imaging (DCI) technique which depends on the complex refractive index of the atom cloud. A semiclassical model allows us to analytically calculate the detuning-dependent refractive index of the system. The predicted Autler-Townes splitting and our experimental measurements are in excellent agreement. DCI provided a quantitative image of the distribution of the excited-state fraction, and compared with on-resonance absorption and blue cascade fluorescence techniques, was found to be experimentally simple and robust.

Effect of biased noise fluctuations on entanglement and statistical properties in a two-photon phase-sensitive laser

Abstract: Analysis of the effects of the biased noise fluctuations on the quantum features and statistical properties of the cavity radiation of the two-photon phase-sensitive laser is presented. It turns out that the biased noise fluctuations that enter the cavity via the vibrations of the wall enhance the atomic coherence responsible for the entanglement of the cavity radiation due to the phase sensitivity of the involved cascade transitions. The two-mode squeezing of the superimposed radiation, entanglement of the cavity radiation, and mean number of the photon pairs are found to increase with the rate at which the atoms are injected into the cavity. The two-mode squeezing increases with the intensity of the biased noise fluctuations, but the mean number of the photon pairs decreases. Even then, it is possible to produce quite intense robust continuous variable entanglement in this scheme which is believed to be a motivation for further practical scrutiny. Moreover, the superimposed radiation exhibits super-Poissonian photon statistics with no finite joint probability for getting more photons in mode $b$ than mode $a$.

Enhanced four-wave mixing by atomic coherence in a Pr3+:Y2SiO5 crystal

Abstract: We experimentally demonstrate an enhanced four-wave mixing (FWM) based on atomic coherence in a Pr3+:Y2SiO5 crystal. By employing coherent population return and fractional stimulated Raman adiabatic passage to prepare maximum atomic coherence in the crystal, an efficient FWM signal can be generated. By measuring the generated FWM signal, the time-dependent atomic coherence is monitored. Such an enhanced FWM in solids may have practical applications in nonlinear optics and laser spectroscopy.

Dark Raman resonances due to Ramsey interference in vacuum vapor cells

Abstract: We investigate the evolution of the atomic coherence between Zeeman sublevels with $\ensuremath{\Delta}{m}_{F}=2$ in the region between pump and probe laser beams, in a Rb vapor cell using the Hanle configuration. The transmission of the probe beam, placed inside the hollow pump beam, shows the narrowing of the Hanle resonance (at external magnetic field $B=0$) by Ramsey fringes whose separation from the central peak depends on the distance between the beams. The temporal evolution of the pump induced alignment is evident from the changes of the probe transmission spectra with increasing the angle between linear polarizations of each beam. Experimental results are in agreement with the results of the model which calculates probe fluorescence by solving time-dependent optical Bloch equations, from the time when atom enters the pump beam to the time when it leaves the probe beam. The results for total excited state populations were averaged over atomic velocities distribution.

Continuous variable entanglement in a coherently pumped correlated emission laser

Abstract: Analysis of the effects of external pumping on the quantum features of cavity radiation of the two-photon correlated emission laser is presented. It turns out that the pumping process induces the atomic coherence responsible for the entanglement of the cavity radiation as well as the correlation between the photon numbers belonging to different modes, despite the arising vacuum fluctuations. The photon number correlation is found to be very close to 2 where the entanglement is relatively better. Moreover, the mean number of photon pairs decreases with the amplitude of the driving radiation which is one of the practical limitations associated with the pumping mechanism. It is also shown that the mean photon number of the radiation in mode b is less than one-third of that in mode a.

Coherence-enhanced entanglement between two atoms at high temperature

Abstract: This paper studies the entanglement properties in a system of two dipole—dipole coupled two-level atoms resonantly interacting with a single-mode thermal field. The results show that, when the temperature of the cavity is high enough (corresponding to the large value of the mean photon number), the entanglement is greatly enhanced due to the initial atomic coherence. These results are helpful for controlling the atomic entanglement by changing the initial parameters of the system.

Externally induced continuous variable entanglement in a correlated emission laser

Abstract: Analysis of the externally induced quantum features of the cavity radiation of the two-photon correlated emission laser is presented. It turns out that the pumping process induces atomic coherence accountable for observing entanglement in the cavity radiation as well as the nonclassical photon number correlation despite the arising vacuum fluctuations. Even though a significant continuous variable entanglement is obtained near threshold in the strong driving limit, increasing the amplitude of the driving radiation more than required for a fixed value of the linear gain coefficient is found to degrade the degree of entanglement. The two-mode squeezing of the superimposed radiation, entanglement of the cavity radiation and mean number of photon pairs increase with the rate at which the atoms are injected into the cavity, but the nonclassical photon number correlation decreases. Moreover, the mean photon number corresponding to the transition from the upper energy level to the intermediate is found to be greater than the transition from the intermediate to the lower. With conceivable practical challenges, it is in principle possible to generate an intense continuous variable entanglement and quantify it via photon count measurements.

XUV coherent Raman superradiance

Abstract: We find that intense short pulses of XUV radiation can be produced by cooperative spontaneous emission from visible or IR laser pulses driving atoms or ions. The process depends on the generation and utilization of atomic coherence as is the case in lasing without inversion. However, the radiation process is not stimulated emission, but is rather cooperative spontaneous emission in the sense of Dicke. More precisely, the many atom mathematics of superradiance has much in common with coherent anti-Stokes Raman scattering. †It is a pleasure to dedicate this article to Professor Lorenzo Narducci: scientist, scholar, savant. All who were touched by him were enriched. The generation of Raman coherence is considered from the perspectives of multi-photon excitation and breaking of adiabaticity. In particular, we find that the advent of an ultrashort (e.g. single cycle) pulse rewards rethinking of conventional (nanosecond) nonlinear optics with new insights.

Transmission and reflection of electromagnetically induced absorption grating in homogeneous atomic media.

Abstract: We theoretically study the transmission and reflection of the probe travelling wave in an electromagnetically induced absorption grating (EIG), which is created in a three-level Λ-type atomic system when the coupling field is a standing wave. Using the system, we show that a photonic stop band can exist on one side away from the resonance point in ultracold atomic gas, while there is an enhanced absorption at resonance and small reflection around it in the thermal atomic gas. Because our method can deal with such two cases, it is helpful to further understand the effects of the Doppler effect on atomic coherence and interference.

Coherence transfer between atomic ground states by the technique of stimulated Raman adiabatic passage.

Abstract: We experimentally and theoretically demonstrate that the atomic coherence can be completely transferred or arbitrarily contributed among the different levels in a four-level atomic (tripod) scheme by a group of coupled pulse sequences. This technique can be applied to the information conversion in slow-light storage, quantum logical gates, and so on, which is based on the atomic coherence effect.

Dynamics of Bragg diffraction in a stored light grating in cold atoms

Abstract: We report on a detailed investigation of the dynamics and the saturation of a light grating stored in a sample of cold cesium atoms. We employ Bragg diffraction to retrieve the stored optical information impressed into the atomic coherence by the incident light fields. The diffracted efficiency is studied as a function of the intensities of both writing and reading laser beams. A theoretical model is developed to predict the temporal pulse shape of the retrieved signal and compares reasonably well with the observed results.

Excitation of atomic coherence at XUV transition by using a far-off resonant two-frequency driving field

Abstract: We study population transfer in a three-level system interacting with a strong off-resonant electromagnetic field consisting of radiation with two optical frequencies. We show that the presence of the second frequency broadens multi-photon resonances and even for higher than 3-photon resonances the probability of excitation weakly depends on optical frequency. We discuss possible applications of results obtained for generation of XUV radiation.

Noise-induced switching via fluctuating atomic coherence in an optical three-level bistable system

Abstract: Noise-induced switching caused by fluctuating atomic coherence is experimentally demonstrated in an atomic optical bistable system consisting of three-level atoms in Λ-type configurations confined in an optical ring cavity. The nonlinear phase as a result of enhanced Kerr nonlinearity induced by atomic coherence in the electromagnetically induced transparency system is fluctuating due to the noise in laser parameters and is primarily responsible for such a switching phenomenon.

Coherent dynamics of self-induced ultraslow light for all-optical switching.

Abstract: Coherent dynamics of slow light for all-optical switching is investigated in a multilevel system of solids for an understanding of self-induced ultraslow light. In an optical population shelving system of a rare-earth doped solid, dynamics of the slow light are presented by using a third optical field controlling shelved atom population. Unlike two-photon coherence-based delayed all-optical switching utilizing electromagnetically induced transparency, the present method relies on one-photon coherence controlling shelved atom population.

Slow light in metamaterial heterostructures

Abstract: A competent method for slowing and completely stopping light, based on wave propagation along an adiabatically tapered negative-refractive-index metamaterial heterostructure, is presented. It is analytically shown that, in principle, this method simultaneously allows for broad bandwidth operation (since it does not rely on group index resonances), large delay-bandwidth products (since a wave packet can be completely stopped and buffered indefinitely) and high, almost 100%, in/out-coupling efficiencies. Moreover, by nature, the presented scheme invokes solid-state materials and, as such, is not subject to low-temperature or atomic coherence limitations. A wave analysis, which demonstrates the halting of a monochromatic field component travelling along the heterostructure, is followed by a corresponding ray analysis that illustrates the trapping of the associated light-ray and the formation of a double light-ray cone (‘optical clepsydra’). This method for trapping photons conceivably opens the way to a multitude of hybrid, optoelectronic devices to be used in ‘quantum information’ processing, communication networks and signal processors, and may herald a new realm of combined metamate rials and slow light research. Keywords: Slow light, metamaterials, negative refractive index, waveguides, adiabatic variation

Electromagnetically induced Bragg reflection with a stationary coupling field in a buffer rubidium vapor cell.

Abstract: An electromagnetically induced Bragg reflection with a stationary coupling field in an Rb vapor cell with a 6.67 kPa neon buffer gas was studied. When the coupling field was spatially modulated as a stationary wave, a transmission reduction of the probe field was observed, while simultaneously a reflected probe field was detected in the backward direction. Instead of absorbing a fraction of the probe laser in the Rb vapor, the modulated electromagnetically induced transparency medium reflected it, corresponding to a Bragg reflection. The spectrum in the 5S½−5P½Λ-type system of 87Rb atoms was investigated as a function of the coupling laser frequency detuning, the stationary coupling laser power, the vapor cell temperature, and the coupling laser power ratio.

Dispersive switching in mesoscopic multistable systems

Abstract: Dispersive switching effect is examined for a mesoscopic system of coherently injected two-level Rydberg atoms into a driven single-mode superconducting cavity. The effect concerns the switching of the output field by simultaneously varying the atomic and cavity detuning parameters at fixed values of the coherent driving field. For certain atomic coherence parameters, symmetric and asymmetric switching diagrams are exhibited, which include one- and two-way process via multistep transitions between output field states.

Selective excitation of atoms and molecules by ultrashort laser pulses

Abstract: Efficient excitation of an ensemble of atoms and molecules to a desired state is of fundamental importance not only for spectroscopy but also for quantum control of molecular dynamics in chemical and biological processes. The latter usually occurs on a much shorter time scale (in the femtosecond range) giving rise to steadily advanced problem how to excite one of the nearby levels, even when they are well within the broad fs pulse spectrum. We propose a new effective method for selective excitation of atoms based on the atomic coherence and discuss a sensitive method to verify experimentally the proposed mechanism using the technique of quantum beating in pump-probe experiments.

Atomic Coherence Spectroscopy in the Paraffin Coated Rb Atom Vapor Cell

Abstract: We investigated the electromagnetically induced transparency (EIT) and the Hanle spectrum in a paraffin coated Rb vapor cell. The EIT spectrum was observed in the $F_g