Showing papers on "Atomic coherence published in 2002"

Transporting and time reversing light via atomic coherence.

Abstract: We study basic issues central to the storage of quantum information in a coherently prepared atomic medium such as the role of adiabaticity. We also propose and demonstrate transporting, multiplexing, and time reversing of stored light.

Efficient frequency up-conversion in resonant coherent media

Abstract: We demonstrate an efficient frequency up-conversion based on generation of large atomic coherence in a cascade system. Two infrared, low power laser fields tuned to the vicinity of the two-photon transition in Rb vapor were converted spontaneously into infrared and blue radiation. Extension of the technique into other spectral regions using highly excited states seems feasible.

Enhancement of Kerr nonlinearity via multi-photon coherence

Abstract: We propose a new method of resonant enhancement of optical Kerr nonlinearity using multi-level atomic coherence. The enhancement is accompanied by suppression of the other linear and nonlinear susceptibility terms of the medium. We show that the effect results in a modification of the nonlinear Faraday rotation of light propagating in an Rb87 vapor cell by changing the ellipticity of the light.

Resonant enhancement of high-order optical nonlinearities based on atomic coherence

Abstract: We show that the effect of coherent population trapping may result in resonant enhancement of ${\ensuremath{\chi}}^{(5)}$ or higher-order nonlinearities. The enhancement is accompanied by suppression of the other linear and nonlinear susceptibility terms. This effect has promise for a realistic scheme of photon phase gates necessary for practical implementation of quantum processing protocols.

Consequences of induced transparency in a double-Λ scheme: Destructive interference in four-wave mixing

Abstract: We investigate a four-state system interacting with long and short laser pulses in a weak probe beam approximation. We show that when all lasers are tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM) field is strongly absorbed. The part that is not absorbed has the exact intensity required to destructively interfere with the excitation pathway involved in producing the FWM state. We show that with this three-photon destructive interference, the conversion efficiency can still be as high as 25%. Contrary to common belief, our calculation shows that this process, where an ideal one-photon electromagnetically induced transparency is established, is not most suitable for high-efficiency conversion. With appropriate phase matching and propagation distance, and when the three-photon destructive interference does not occur, we show that the photon flux conversion efficiency is independent of probe intensity and can be close to 100%. In addition, we show clearly that the conversion efficiency is not determined by the maximum atomic coherence between two lower excited states, as commonly believed. It is the combination of phase matching and constructive interference involving the two terms arising in producing the mixing wave that is the key element for the optimized FWM generation. Indeed, in this scheme no appreciable excited state is produced, so that the atomic coherence between states |0〉 and |2〉 is always very small.

Controlling light by light with three-level atoms inside an optical cavity.

Abstract: We present our experimental demonstration of controlling the cavity output intensity of one laser beam with the intensity of another laser beam in a composite system consisting of a collection of three-level ?-type rubidium atoms and an optical ring cavity. When the intensity of the controlling beam is modulated with a square waveform, the cavity output power switches on and off (with a distinction ratio better than 20:1) between two steady-state values. This all-optical switching effect is the result of combined absorption and enhanced Kerr nonlinearity near resonance in such three-level atomic systems because of atomic coherence and can find applications in optical communication and optical computation.

Controlling the cavity field with enhanced Kerr nonlinearity in three-level atoms

Abstract: We experimentally demonstrate cavity field control by another laser beam with three-level \ensuremath{\Lambda}-type atoms inside an optical ring cavity. By adjusting the frequency detuning of the controlling beam (coupled to one of the atomic transitions), the intensity of the cavity field interacting with another atomic transition is switched on and off. Such all-optical switching between two steady states is caused by the enhanced Kerr nonlinearity due to atomic coherence in such system.

Dependence of enhanced Kerr nonlinearity on coupling power in a three-level atomic system.

Abstract: We study the enhanced Kerr-nonlinear coefficient in a three-level Λ-type atomic system for various coupling-beam powers. The Kerr-nonlinear coefficient behaves very differently in the strong and the weak coupling power regions and changes sign when the coupling or probe frequency detuning changes sign. Comparisons of Kerr-nonlinear coefficients as functions of probe frequency detuning, coupling power, and coupling frequency detuning are presented.

Atomic coherence induced Kerr nonlinearity enhancement in Rb vapour

Abstract: The Kerr nonlinear index of refraction for rubidium atoms is measured by using an optical ring cavity with and without electromagnetically induced transparency (EIT). Significant enhancement and inhibition of the Kerr nonlinear index is observed near resonance with EIT. The nonlinear index of refraction is measured as functions of probe and coupling frequency detunings, respectively, with and without the presence of EIT. A simple theoretical calculation including Doppler broadening is presented and is found to be in good qualitative agreement with the experimentally measured results.

From laser-induced line narrowing to electromagnetically induced transparency in a Doppler-broadened system

Abstract: The laser-induced line narrowing effect in Doppler broadened systems was discovered thirty years ago. We have revisited this effect to determine its role for dense gases where recent experimental studies have found many intriguing atomic coherence effects. Using the density matrix approach, we study the width of electromagnetically induced transparency under different regimes of broadening.

Transmission Probability of an Ultracold Atom in the Presence of Atomic Coherence

Abstract: We investigate the transmission probability of an ultracold V-type three-level atom passing through a micromaser cavity, in the presence of atomic coherence which is established by a coherent driving field. We show that the transmissibility of this micromaser system with the atomic coherence is better than that of the ordinary micromaser system without atomic coherence. When the driving field is strong enough, for any cavity length the ultracold atom can pass through the micromaser cavity freely.

Light Controlling Light with Enhanced Kerr Nonlinearity

Abstract: The Kerr nonlinear index of refraction of a three-level atomic medium can be greatly enhanced by atomic coherence induced by two laser beams. Such electromagnetically induced transparency (EIT) media can be used to generate efficient nonlinear optical processes at low light intensities. Control and enhancement of Kerr nonlinearity in EIT media can be useful in all-optical communication, all-optical computing, and quantum information processing.

The Photo‐Carnot Cycle: The Preparation Energy for Atomic Coherence

Abstract: We consider a Carnot cycle engine in which the working medium is a photon gas inside a cavity with perfectly reflecting walls. The thermal baths responsible for supplying heat to the radiation field is a stream of three‐level atoms. The phase of the atomic coherence provides a new and interesting control parameter. Here we discuss the questions pertaining to the preparation energy of atomic coherence with an explicit calculation of the associated entropy change.

Resonant enhancement of refractive index in a cascade scheme

Abstract: The refractive index of coherently driven rubidium vapour is experimentally investigated in a three-level cascade con® guration using a selective reection technique. The maximum measured resonant change in the refractive index is ¢n ' 0:1. The selective reection is accompanied by a four-wave-mixing process that can reach π90% eA ciency.

Maximal atomic coherence via selective trapping of dressed states

Abstract: We consider strong-field index enhancement in three-level systems in which there is a strong spontaneous decay from an auxiliary level into either of two states that the probe field couples. A control field is detuned resonant with the transition between the auxiliary level and one of the dressed states produced by the strong probe field. It is shown that it is possible to achieve maximal atomic coherence, which characterizes an ultralarge index of refraction and vanishing absorption. This scheme is based on selective dressed population trapping, which is established by relying on the control field and the strong decay to transfer population from one dressed state to another through the auxiliary level. An advantage of the present scheme is that the conditions for its realization are accessible experimentally.

Generation of higher-order atomic dipole squeezing in a high-Q micromaser cavity. (VI). Atomic damping by a squeezed vacuum

Abstract: In our previous serial works, we investigated the generation of higher-order atomic dipole squeezing (HOADS) in a high-Q micromaser cavity. In this paper, we continue to study HOADS in this system but consider the effect of atomic damping by a squeezed vacuum in the absence of an external field. The corresponding master equation is introduced and its solutions for an initial SU(2) coherent state of the atom are presented. In the absence of the atomic damping, we have shown that stable and permanent HOADS can be generated by properly preparing the atom in certain SU(2) coherent state. We demonstrate the destructive effect of atomic damping on HOADS by changing the initial atomic coherence or the average number of the reservoir photons. The relation between HOADS and second-order atomic dipole squeezing (SOADS) is discussed. It is found that the atomic damping has more destructive effects on SOADS than on HOADS.

Emission probability and photon statistics of a coherently driven mazer

Abstract: The idea of a mazer is put forward with particular reference to the question of the driving-induced atomic coherence, which is established by a coherent driving field. The interaction of a quantum cavity field and an ultracold V-type three-level atom in which two levels are coupled by a coherent driving field, is derived. Its general quantum theory is established and the atomic emission probability and photon statistics are calculated and analysed. It is found that the mazer based on this driving-induced atomic coherence shows new features. There is a non-vanishing probability for the atom emitting a photon in the cavity even when the resonance condition is not fulfilled (here the resonance condition means that the cavity length is an integer multiple of half the atomic de Broglie wavelength). Under the resonance condition, the atomic emission probability has two sets of resonance peaks. For a very strong coherent driving field, the emission of the atom can be forbidden. As to the photon statistics, when the driving field is not very strong, the driving-induced atomic coherence reduces the photon number fluctuations of the cavity field. The photon statistics exhibits strong sub-Poissonian behaviour. In the region considered here, it can even be sub-Poissonian for any cavity length. However, when the driving field is too strong, the sub-Poissonian property may disappear.

Emission Probability of the Cascade Three-Level-Atom Mazer with Injected Atomic Coherence

Abstract: We investigate the effects of the injected atomic coherence on the atomic emission probability of the micromaser injected with ultracold cascade three-level atoms by considering that the atoms are initially in the coherent superposition states of the two upper levels We show that there is no interference between the transitions from the two upper levels to the lowest level In the large atom-field-detuning case, the atomic emission probability decreases as the coherent parameter increases In the zero atom-field-detuning case, the atomic emission probability has three sets of resonance peaks The reason for these results has been explained

Observations of absorptive photon switching and suppression of two-photon absorption in cold atoms

Abstract: Atomic coherence and interference in an atomic medium exhibiting electromagnetically induced transparency may lead to enhancement or suppression of nonlinear susceptibilities. Absorptive photon switching has been observed by constructive quantum interference, which is based on the enhanced third-order, nonlinear absorption in a four-level system. In a different four-level system, suppression of the two-photon absorption by destructive quantum interference has been observed. Experiments were carried out on 87Rb atoms cooled and confined in a magneto-optical trap and the experimental results agree with theoretical calculations of simple four-level model systems.

Generation of higher-order atomic dipole squeezing in a high-Q micromaser cavity. (VII). Entangled two-mode coherent states

Abstract: In our preceding paper V, we investigated the generation of higher-order atomic dipole squeezing (HOADS) in a nondegenerate two-photon Jaynes–Cummings model (NTPJCM) in the presence of Stark shift. In this paper, we continue to study HOADS in this model but focus on the specific cases that the radiation field is initially prepared in a two-mode entangled state (e.g., two-mode squeezed vacuum state, two-mode Perelomov and Barut–Girardello coherent states). It is found that increasing the fixed difference in the photon numbers of the two-mode entangled states of the radiation field could decrease the squeeze duration and shorten the squeeze period, and the detuning may lead to much effective HOADS by properly adjusting certain value. In general, the Stark shift has a destructive effect on HOADS, but the combined effect of the detuning and Stark shift could increase the squeeze duration and lead to regular and periodical HOADS pattern. The influence of atomic coherence on HOADS is also examined in detail.

Electromagnetically Induced Transparency; Writing, Storing, and Reading Short Optical Pulses

Abstract: The spatiotemporal propagation dynamics of a weak probe pulse in an optically dense medium of three-level atoms is studied in the adiabatic approximation under conditions of electromagnetically induced transparency. The atomic coherence induced at the dipole-forbidden transitions is found to be spatially localized. This effect is used for the analysis of the reversible writing (reading) of short optical pulses. The method of pulse time reversal is suggested.

Enhanced coherent emission aided by an incoherent process

Abstract: Enhanced emission of a coherent signal generated by the nonlinear process ωf+ωb-ωp has been found in an incoherently pumped four-level system. It is shown that finite (large) atomic coherence can be generated by (incoherent) pumping of the intermediate states, which removes the destructive interference effects that otherwise inhibit coherent emission of signals.

Strong-field index enhancement via incoherent excitation

Abstract: We show that it is possible, in principle, to employ an incoherent field as a control field to achieve maximal atomic coherence for a probe field, which characterizes ultralarge index of refraction and vanishing absorption. For this purpose we consider a three-level V system in which there are two dipole transitions, of which one is weak and the other is strong. The strong probe field is applied to the weak transition and the incoherent field drives the strong transition. For such an arrangement, selective dressed-population trapping, which corresponds to maximal atomic coherence, is established by relying on the incoherent field and the spontaneous relaxation to transfer population from one dressed state to the other. This scheme as an extreme example explains that strong-field index enhancement based on selective dressed-population trapping is resistant to the control field fluctuations.

Multiphoton transitions in a colored vacuum: coupling of the ac Stark shift with spontaneous decay and the Lamb shift

Abstract: In free space, where the vacuum mode density varies relatively slowly in the spectral region of an atomic transition, an atom’s real-field and vacuum interactions are effectively decoupled. Consequently, spontaneous decay and the Lamb shift are, for all practical purposes, independent of any real-field atomic perturbation. However, in a colored vacuum (i.e., a low-Q cavity) the vacuum mode density can change significantly in the vicinity of an atomic transition, so that a Stark shift will alter an atom’s vacuum environment and thereby couple real-field and vacuum effects. Since the ac Stark shift is an inherent aspect of multiphoton processes, this coupling is unavoidable for highly nonlinear field–atom interactions that occur in cavities. Here we consider this effect for 3+2 resonance-enhanced multiphoton ionization of xenon.

Evidence of dark state stabilization via the quantum Zeno effect

Abstract: Summary form only given. We have studied the decay of the atomic coherence on atoms initially prepared in a dark state (DS) when the coherence (Raman) resonance condition between the atomic system and the light field is suddenly removed. Under this condition, the DS is no longer stationary and its temporal evolution reveals the decay of the atomic coherence. Since in our experiments the DS is created in a ground level and dephasing collisions are negligible, the only possible relaxation channel is through the coupling to the light field. We have studied the influence of the light field intensity on the decay of the atomic coherence. We perform a Hanle type experiment on /sup 87/Rb atoms in a vapor cell.

All-optical switching via enhanced Kerr nonlinearity in three-level atoms

Abstract: Summary form only given. The reduced absorption and enhanced nonlinearity in a three-level electromagnetically induced transparency (EIT) system are very important in achieving an effective all-optical switching action. The Kerr-nonlinear index of refraction of a three-level EIT medium can be greatly enhanced near resonance due to atomic coherence induced in such a system in comparison with a two-level atomic system, as we have shown by using an optical ring cavity. Both the magnitude and sign of the Kerr-nonlinear index change dramatically for different frequency detunings of the controlling (or coupling) or cavity (or probe) field. Bistability and dynamic instability occur when such a three-level /spl Lambda/-type EIT system is placed inside an optical ring cavity and the controlling and cavity input intensities to exceed certain threshold values. In this paper, we present a recent experimental demonstration of controlling the cavity output intensity by modulating the frequency detuning of the controlling beam.

Enhanced Kerr nonlinearity for self-action via atomic coherence in a four-level atomic system

Abstract: Enhancement of optical Kerr nonlinearity for self-action by electro-magnetically induced transparency in a four-level atomic system including dephasing between the ground states is studied in detail by solving the density matrix equations for the atomic levels. We discern three major contributions, from energy shifts of the ground states induced by the probe light, to the third-order susceptibility in the four-level system. In this four-level system with the frequency-degenerate probes, quantum interference amongst the three contributions can, not only enhance the third-order susceptibility more effectively than in the three-level system with the same characteristic parameters, but also make the ratio between its real and imaginary part controllable. Due to dephasing between the two ground states and constructive quantum interference, the most effective enhancement generally occurs at an offset that is determined by the atomic transition frequency difference and the coupling Rabi frequency.

Electromagnetically induced transparency: record, storage and recover of short pulses

Abstract: Summary form only given. In this report the propagation of a weak probe pulse in an optically dense medium in presence of strong coupling pulse, resonant to conjugated transition, is investigated. We consider the case when at the medium entrance both pulses have the identical shape, but duration of coupling pulse is greater than duration of probe pulse. The intensity of the probe pulse is considered much less than the intensity of the coupling one. The features of spatial and temporal evolution of the probe pulse and the atomic coherence are analyzed at different relations between pulse duration and relaxation time of the medium.