Showing papers on "Atomic coherence published in 2010"

Atomic optical bistability in two- and three-level systems: perspectives and prospects

Abstract: The atomic optical bistability (AOB) in multi-level systems has advantages over the two-level system as absorption, dispersion, and nonlinear optical properties of one optical field coupled to one atomic transition can be greatly modified by other optical fields coupled to other connected nearby atomic transitions in multi-level atomic systems due to induced atomic coherences. By making use of such changes in linear and nonlinear optical properties around resonance, which are related to the electromagnetically induced transparency (EIT), it is easy to manipulate and control nonlinear optical processes in the multi-level atomic systems. We review earlier two-level AOB theory, as well as experiments and some recent studies that made use of multi-level atomic EIT systems in achieving controls of nonlinear optical processes such as instabilities and stochastic resonance associated with the coupled atom–cavity systems.

Evidence of Autler–Townes splitting in high-order nonlinear processes

Abstract: We demonstrate Autler–Townes (AT) splitting of four-wave mixing in an electromagnetically induced transparency window, which results from the destructive interference between a three-photon process and a five-photon process. The primary and secondary AT splittings are achieved via induced atomic coherence in a four-level Y-type atomic system. Theoretical calculations fit well with the experimentally measured results. Such controlled multichannel splitting of nonlinear optical signals can have potential applications in optical communication and quantum information processing.

Fast light in atomic media

Abstract: Atomic media have played a major role in studies of fast light. One of their attractive features is the ability to manipulate experimental parameters to control the dispersive properties that determine the group velocity of a propagating light pulse. We give an overview of the experimental methods, based on both linear and nonlinear atom–light interaction, that have produced superluminal propagation in atomic media, and discuss some of the significant theoretical contributions to the issues of pulse preservation and reconciling faster-than-light propagation and the principle of causality. The comparison of storage of light, enhanced Kerr nonlinearity and efficient wave mixing processes in slow and fast light atomic media illustrates their common and distinct features.

Modulated vortex solitons of four-wave mixing

Abstract: We experimentally demonstrate the vortex solitons of four-wave mixing (FWM) in multi-level atomic media created by the interference patterns with superposing three or more waves. The modulation effect of the vortex solitons is induced by the cross-Kerr nonlinear dispersion due to atomic coherence in the multi-level atomic system. These FWM vortex patterns are explained via the three-, four- and five-wave interference topologies.

Carrier-envelope phase effect on atomic excitation by few-cycle rf pulses.

Abstract: We present an experimental and theoretical study of the carrier-envelope phase effects on population transfer between two bound atomic states interacting with intense ultrashort pulses. Radio frequency pulses are used to transfer population among the ground state hyperfine levels in rubidium atoms. These pulses are only a few cycles in duration and have Rabi frequencies of the order of the carrier frequency. The phase difference between the carrier and the envelope of the pulses has a significant effect on the excitation of atomic coherence and population transfer. We provide a theoretical description of this phenomenon using density matrix equations. We discuss the implications and possible applications of our results.

Transformation of electromagnetically induced transparency into enhanced absorption with a standing-wave coupling field in an Rb vapor cell

Abstract: We present the transformation of electromagnetically induced transparency (EIT) into narrow enhanced absorption with an on-resonant standing-wave coupling field in the 5S1/2-5P1/2 transition of the Λ-type system of 87Rb atoms. When a coupling laser field was changed from a travelling-wave to a standing-wave that was made by adding a counter-propagating LC laser, the transmittance spectrum of the LP laser transformed the typical EIT into dramatically enhanced absorption, and a Bragg reflection signal was generated by the periodic modulation of atomic absorption. The reflected probe laser corresponding to a Bragg reflection was measured to be approximately 11.5% of the power of the incident probe laser. We analyzed the enhanced absorption signal and Bragg reflection spectrum as a function of the power and frequency detuning of the coupling laser.

Adiabatic splitting, transport, and self-trapping of a Bose-Einstein condensate in a double-well potential

Abstract: We show that the adiabatic dynamics of a Bose-Einstein condensate (BEC) in a double-well potential can be described in terms of a dark variable resulting from the combination of the population imbalance and the spatial atomic coherence between the two wells. By means of this dark variable, we extend, to the nonlinear matter-wave case, the recent proposal by Vitanov and Shore [Phys. Rev. A 73, 053402 (2006)] on adiabatic passage techniques to coherently control the population of two internal levels of an atom or molecule. We investigate the conditions to adiabatically split or transport a BEC as well as to prepare an adiabatic self-trapping state by the optimal delayed temporal variation of the tunneling rate via either the energy bias between the two wells or the BEC nonlinearity. The emergence of nonlinear eigenstates and unstable stationary solutions of the system as well as their role in the breaking down of the adiabatic dynamics is investigated in detail.

Entanglement between collective fields via atomic coherence effects

Abstract: We explore the quantum entanglement between two collective fields via atomic coherence effects. For three-level atoms in $\mathsf{V}$ configuration driven by two applied fields on two-photon resonance, one coherent superposition of the excited states is not excited, which is the counterpart of coherent population trapping. The coherence-induced depopulation makes two cavity fields in each collection combine into a quantum-beat, i.e., equivalently, the difference mode of the two components decouples from the driven atoms. The two sum modes, when they are arranged in the four-wave mixinglike interactions, can be prepared in Einstein-Podolsky-Rosen entangled state. Correspondingly, any two individual fields from different collective modes are entangled with each other. Furthermore, the effects of thermal reservoir and laser linewidths are discussed, and a generalization is given to the case in which each quantum beat involves more than two modes.

Tunable double photonic bandgaps in a homogeneous atomic medium

Abstract: Double photonic bandgaps (PBGs) can simultaneously appear when double dark resonances in uniform cold atoms are spatially modulated by a resonance standing-wave. Theoretical calculations show that variable and efficient coherent optical control of the PBGs can be achieved by modulating the coupling field and standing-wave. The structures of double PBGs induced by the atomic coherence effect are better than those obtained in the photonic crystal heterostructures. We anticipate that this scheme has potential applications in optical networks for dual-channel all-optical switching or a dual-frequency optical Bragg reflector.

Complex spatial structures due to atomic coherence

Abstract: The coupling of diffraction and optical coherence of a three-level medium in an optical cavity is considered. Self-organisation and pattern formation due to a combination of electromagnetically induced transparency, close-to-resonance Kerr effects and diffraction is demonstrated. Self-focusing in the peaks of the resulting Turing pattern leads to localised amplification of the input pump. Spatial dissipative solitons with controllable position, dispersion and coherence are also described.

Adiabatically driven frequency conversion towards short extreme-ultraviolet radiation pulses

Abstract: We present an experimental demonstration of sum-frequency generation toward tunable picosecond, broadband radiation pulses in the extreme-ultraviolet regime, supported by adiabatically prepared atomic coherences. We drive a two-photon transition in a dense medium of xenon atoms with a long (nanosecond) pump laser pulse at the Fourier-transform-limited bandwidth and with a wavelength of 225 nm. The frequency of the pump pulse is slightly detuned from exact two-photon resonance. In this configuration, the medium is adiabatically driven by a process of coherent population return. The adiabatic passage process generates a maximal coherent superposition of two quantum states with large energy spacing. An additional, short (picosecond) probe laser pulse at a wavelength of 540 nm beats with the maximal atomic coherence and generates a short (picosecond) signal radiation pulse at 93 nm. As compared to conventional (diabatic) frequency conversion, the adiabatically driven maximal atomic coherence yields enhanced conversion efficiency and significantly enhanced stability.

Emission of photon echoes in a strongly scattering medium

Abstract: We observe the two- and three-pulse photon echo emission from a scattering powder, obtained by grinding a Pr$^{3+}$:Y$_2$SiO$_5$ rare earth doped single crystal. We show that the collective emission is coherently constructed over several grains. A well defined atomic coherence can therefore be created between randomly placed particles. Observation of photon echo on powders as opposed to bulk materials opens the way to faster material development. More generally, time-domain resonant four-wave mixing offers an attractive approach to investigate coherent propagation in scattering media.

Coherence-Enhanced Entanglement Induced by a Two-Mode Thermal Field

Abstract: Considering two identical two-level atoms interacting with two mode thermal field through a nondegerate two-photon process, we study the entanglement dynamics between two atoms when the atomic coherence exists. It shows that the entanglement is dependent on the initial atomic states, and is greatly enhanced due to atomic coherence as compared with the case when the atomic coherence is ignored. The results also show that the entanglement can be controlled by changing the relative phases and the amplitudes of the polarized atoms.

All-optical pump-and-probe detection of two-time correlations in a Fermi gas

Abstract: We propose an all-optical scheme to probe the dynamical correlations of a strongly interacting gas of ultracold atoms in an optical lattice potential. The proposed technique is based on a pump-and-probe scheme: a coherent light pulse is initially converted into an atomic coherence and later retrieved after a variable storage time. The efficiency of the proposed method to measure the two-time one-particle Green function of the gas is validated by numerical and analytical calculations of the expected signal for the two cases of a normal Fermi gas and a BCS superfluid state. Protocols to extract the superfluid gap and the full quasiparticle dispersions are discussed.

Gravitational wave detection with single-laser atom interferometers

Abstract: We present a new general design approach of a broad-band detector of gravitational radiation that relies on two atom interferometers separated by a distance L. In this scheme, only one arm and one laser will be used for operating the two atom interferometers. We consider atoms in the atom interferometers not only as perfect inertial reference sensors, but also as highly stable clocks. Atomic coherence is intrinsically stable and can be many orders of magnitude more stable than a laser. The unique one-laser configuration allows us to then apply time-delay interferometry to the responses of the two atom interferometers, thereby canceling the laser phase fluctuations while preserving the gravitational wave signal in the resulting data set. Our approach appears very promising. We plan to investigate further its practicality and detailed sensitivity analysis.

Bragg diffraction of interacting Bose-Einstein condensates

Abstract: We develop a theoretical model to investigate the Bragg diffraction of an elongated interacting Bose-Einstein condensate which is illuminated by a pair of laser beams. We find that the mean-field effect resulting from the atomic interaction plays an important role in modifying the atomic coherence. Our results show that both the repulsive and attractive interactions would dampen the momentum oscillation of the condensate; they establish surprisingly distinguishable equilibria for the atomic occupations among different diffraction orders. We also give an experimental proposal to observe this phenomenon.

CPT transients induced by rapid changes in laser polarization: validation of a semi-empirical model

Abstract: In ultraminiature atomic physics (UAP), where the goal is to perform low-power, 'chip-scale' precision spectroscopy, stochastic-field/atom interactions can play a primary role in defining the limits of sensitivity. Unfortunately, the transient responses of a quantum system to random changes in the amplitude, phase or polarization of a resonant field are not well understood, forming the basis of what has come to be known as the stochastic-field/atom interaction problem. As the first step in understanding the more complicated stochastic problem, the present work considers the response of a coherent-population-trapping (CPT) signal to a sudden, step-function change in laser polarization. We find that the transient behaviour depends on both the redistribution of atomic population among the atom's Zeeman sublevels and the regeneration of atomic coherence between these sublevels. Despite the complicated nature of the dynamics, we develop and experimentally validate a semi-empirical, reasonably intuitive model of the CPT transient and demonstrate that in the 'typical' CPT signal the polarization-induced transients are dominated by redistribution of atomic population among the Zeeman sublevels. Further, the amplitudes of the polarization-induced transients are relatively large and could potentially 'swamp' the CPT signals of interest for UAP.

Light-matter entanglement via dark-state resonances

Abstract: We show that dark-state resonances can be a fundamental mechanism for the entanglement between light and matter. While two optical fields trap an ensemble of three-level {Lambda} atoms into the dark state, the coherence is created between the two metastable states, and the two fields undergo no absorption. The trapped atoms behave like a coherently coupled two-party reservoir and act on the two fields. As a result, Einstein-Podolsky-Rosen entanglement is obtainable between the atomic ensemble and one collective field under proper parameter conditions. Long-lived atomic coherence and no need of nonclassical input light are the advantages for quantum communications between light and matter based on the present mechanism.

Spontaneously created entanglement between two atoms

Abstract: Spontaneous emission as a potential tool for creation of entanglement between two atoms is investigated. We assume that the atoms are coupled to the same environment and study entanglement engineering between the atoms and its transfer between different states. The role of the atomic coherence induced by spontaneous emission will be explored which, in contrast to what is generally believed, can create entanglement between initially unentangled atoms. We quantify entanglement by the concurrence and find that it exhibits threshold properties that can lead to interesting noncontinuous phenomena of sudden birth and sudden death of entanglement. In addition, we consider the mechanism involved in creation of entanglement between distant atoms coupled to a single-mode cavity field. We include a possible variation of the coupling constants between the atoms and the cavity mode with location of the atoms in a standing-wave cavity mode. Effectively, we engineer two coupled atoms whose the dynamics are analogous to that of interacting and collectively damped two nonidentical atoms. We illustrate the interesting result that spatial variations of the coupling constants can lead to a stationary entanglement between the atoms. We explain this effect in terms of the trapping phenomenon of atomic population in a non-decaying entangled state.

Commentary: Controlling electromagnetic metamaterials

Abstract: The last two decades have been witness to two exciting and independent developments that have forever changed our conventional view of how light interacts with matter. One relates to coherent control via quantum interference, wherein the possibility of making an otherwise opaque medium transparent [1], now known as electromagnetically induced transparency (EIT), set off intense research activity. EIT essentially requires careful creation of atomic coherence, that results in diverse effects varying from almost freezing light in its tracks (slow light) to freezing atoms to nanoKelvin temperatures via velocity-selective coherent population trapping. The second development relates to metamaterials whose origins are very classical in nature. In electromagnetics, these designer materials were originally proposed for realizing a super-lens wherein the evanescent field becomes the work horse that accords sub-wavelength resolution in imaging [2]. Since then a variety of metamaterials have been proposed, where even the propagating fields can be dramatically controlled, as in electromagnetic cloaks wherein the fields are maneuvered around an obstacle so as to make it invisible. The biggest technological contraints in realizing large-scale device applications of metamaterials have been two. The first is the large dissipation associated with an inherently resonant phenomenon. The second arises due to the very design of metamaterial; once the metamaterial structures (inclusions) are fabricated, they offer little maneuverability in terms of the operating frequency. However, both EIT and metamaterials have truly lifted the tedium associated with the usual classical linear phenomena involving light. It is commonly believed this is just the beginning of a long journey, where our inherent drive to control gainfully these and many other wondrous effects will be the prime mover of future developments. The marriage of these two diverse developments is highlighted here, with a word of caution: one can only naively guess the surprises this relationship will bring forth. In order to achieve a composite material combining the attributes of EIT and metamaterials, one simple design involves immersing the metamaterial in a dilute atomic gas whose frequency-selective absorption can be exploited to manipulate the metamaterial response [3]. Furthermore, a combination of light fields accords extra control over the metamaterial via the absorption and dispersion of the atomic gas through the atomic coherence (quantum) route, based on effects like EIT. The price of working at near-resonant conditions is the large dispersion with frequency accompanied by large loss. EIT-based control exploits the large frequency-dispersion and yet provides substantially decreased loss which is even lower than the metallic losses in a narrow-bandwidth regime. The large variation of the refractive index of the EIT medium results in the freezing of currents in the metallic inclusions of the metamaterial, thereby lowering loss. The most critical issues that govern this alliance arise from the very nature of the two partners. EIT is a quantum phenomenon, whereas metamaterials are described classically. Quantum phenomena are extremely susceptible to the surroundings, and using a solid or liquid medium severely restricts the performance of the quantum partner. For example, rareearth ions implanted in crystals have been shown to exhibit EIT only at extremely low temperatures where the phonon noise is sufficiently suppressed such that the ground-state

Two-mode entanglement and squeezing in a three-level cascade atomic system

Abstract: The generation and evolution of entanglement and squeezing for the two-mode fields are discussed in a cascade three-level atomic system. Two photons of a strong external pump field induce atomic coherence between the top and bottom levels. The dynamics of this system in the presence of cavity losses is studied. It is shown that entanglement and squeezing between the two modes increase initially, then decrease and eventually disappear. The periods of them are extended with the increasing of the pump field intensity. The effects of the cavity losses on squeezing and entanglement are also discussed.

Manipulating atomic coherence and interference in a coupled atom-cavity system

Abstract: Collective coupling of multiple atoms with a cavity mode produces two normal modes that are separated in energy by Vacuum Rabi splitting. We show that quantum coherence and interference can be produced by a control laser that couples the atoms confined in the cavity mode from free space, which leads to suppression of the normal mode excitation, or polariton excitation of the cavity-atom system. The control laser splits the normal mode of the cavity-atoms system and opens two excitation channels. The destructive quantum interference between the two channels renders the cavity-atoms system opaque to the light coupled into the cavity mode. We demonstrate suppression of the normal mode (polariton) excitation by the destructive quantum interference in an experiment with cold Rb atoms confined in an optical cavity.

Repetitive interrogation of 2-level quantum systems

Abstract: Trapped ion clocks derive information from a reference atomic transition by repetitive interrogations of the same quantum system, either a single ion or ionized gas of many millions of ions. Atomic beam frequency standards, by contrast, measure reference atomic transitions in a continuously replenished “flow through” configuration where initial ensemble atomic coherence is zero. We will describe some issues and problems that can arise when atomic state selection and preparation of the quantum atomic system is not completed, that is, optical pumping has not fully relaxed the coherence and also not fully transferred atoms to the initial state. We present a simple two-level density matrix analysis showing how frequency shifts during the state-selection process can cause frequency shifts of the measured clock transition. Such considerations are very important when a low intensity lamp light source is used for state selection, where there is relatively weak relaxation and re-pumping of ions to an initial state and much weaker ‘environmental’ relaxation of the atomic coherence set-up in the atomic sample.

Changes in the statistical and quantum features of the cavity radiation of a two-photon coherent beat laser due to phase fluctuation

Abstract: Detailed derivation of the master equation and the corresponding time evolution of the cavity radiation of a coherent beat laser when the atoms are initially prepared in a partial coherent superposition is presented. It turns out that the quantum features and intensity of the cavity radiation are considerably modified by the phase fluctuation arising due to the practical incapability of preparing atoms in the intended coherent superposition. New terms having an opposite sign with the contribution of the driving radiation emerged in the master equation. This can be taken as an indication for a competing effect between the two in the manifestation of the nonclassical features. This, on the other hand, entails that there is a chance for regaining the quantum properties that might have lost due to faulty preparation by engineering the driving mechanism and vice versa. In light of this, quite remarkably, the cavity radiation is shown to exhibit nonclassical features including two-mode squeezing and entanglement when there is no driving and if the atoms are initially prepared in a partial maximum atomic coherence superposition, contrary to earlier predictions for the case of perfect coherence.

Tripartite Entanglement via Microwave Driven Atomic Coherence

Abstract: We propose a new scheme to achieve the tripartite entanglement based on the standard criteria [Phys. Rev. A 67 (2003) 052315] in a inverse-tripod atomic system. In our scheme, the atomic coherence is introduced by two microwave fields which drive the upper three levels of atom. By numerically simulating the dynamics of system, we investigate the generation and evolution of entanglement in the presence of atom and cavity decay. As a result, the present research provides an efficient approach to achieve fully tripartite entanglement with different frequencies and initial states for each entangled mode, which may have impact on the progress of multicolored multi-notes quantum information networks.

Applications of Coherent Raman Scattering

Abstract: Recent explorations based on the concept of molecular coherence have lead to exciting developments, extending the ideas of coherence from atomic physics to more complex, molecular systems. Atomic coherence lies at the core of such fascinating phenomena as electromagnetically induced transparency, ultra- slow light propagation, and lasing without inversion, which all have been subjects of research and debate over decades. In turn, macroscopic molecular coherence allows broadband collinear generation of Raman sidebands, opening possibilities for compression of optical sub-cycle pulses, and for non-sinusoidal field synthesis; increased coherence also enables improvements in optical detection and sensing applications.

A rapid inspection of atomic interference using superfluorescent picosecond pulses

Abstract: The conventional method to measure atomic interference is usually based on incoherent processes which can be scaled by nano or microseconds. However, atomic interference may be studied in association with coherent process such as superfluorescence. Producing superfluorescent picosecond pulses in 87Rb vapor pumped by ∼100 fs laser pulses, we report an observation of quantum beat due to D-lines. The relative delay of the superfluorescent pulses is measured by the streak camera with picosecond time resolution, as a function of time interval between input pulses, which also enables us to study atomic interference.

Role of cavity induced decay-interference effect on vacuum-Rabi splitted spectrum

Abstract: We have shown that an ultranarrow dark region can be associated with the feature of significant amount of vaccuum-Rabi splitting in the absorption spectrum of a weak coherent field probing a two-level atom strongly coupled to two degenerate cavity-modes with preselected polarizations. The cavity-induced transparency is produced by the decay-interference of the two independent cavity modes interacting with the same reservoir. It has been shown that the resonant probe transparency is independent of the individual decay rates of the cavity modes. The present model exhibits the possibility of obtaining the narrow dark transparency associated with the significant amount of Rabi splitting in the absorption spectrum with the appreciable value of the cavity-losses.