Dark-line in optically-thick vapors: inversion phenomena and line width narrowing

An analysis of the phenomenon of coherent population trapping as observed by means of the dark state in the case of optically thick vapors in alkali metal atoms is presented. Very different behaviors are predicted for the observation of the dark state in the fluorescence and transmission spectra of the same atomic sample when the optical length is not negligible. Among other effects, the dark line observed in the fluorescence signal may look inverted appearing as a bright line, while in the transmission signal a narrowing of the line width resonance line is observed for an increase of the atomic density. In the pure three-level scheme a subnatural width is predicted. These effects, related to the electromagnetically induced transparency phenomenon, are readily observed in experiments, which are found to be in excellent agreement with the theory developed. Received 28 March 2001 and Received in final form 3 October 2001     

A dense collection of two-level atoms embedded in a dielectric medium in a ring cavity

The hysteresis behavior of a dense collection of two-level atoms embedded in a linear dielectric medium in an optical cavity is investigated theoretically in terms of a parametric formulation method under the thin-sample approximation. The relation of the output intensity to the input intensity is obtained in an explicit formula. The numerical results show that the hysteresis behavior and the cavity transmission are different from those in vacuum because of the local-field enhancement and the local cooperative decay effect. Received: 27 June 2000 / Revised version: 11 September 2000 / Published online: 10 January 2001     

High-resolution atomic channeling using. velocity-selected atomic beam

High-resolution atomic channeling using velocity-controlled atoms may be able to overcome precision limitations of the conventional atom lithography. We have experimentally clarified the dependence of line width and contrast of atomic patterns in the channeling region on the velocity spread of the atomic source for the first time. Thermal or velocity-selected atomic beams prepared with a one-dimensional magneto-optical trap were employed as the atomic sources. We investigated the channeling characteristics by measuring the frequency shifts of the atomic absorption spectra in an intense standing wave light field. As a result, we can show that narrower line width and higher contrast atomic patterns are obtained as the velocity spread becomes narrower. An atomic pattern with an estimated line width of 57 nm was generated when the velocity spread of the atomic source was almost 50 m/s, that is, 1/6 that of the thermal beam. Received: 16 June 2001 / Published online: 7 November 2001     

Simplified approach to double jumps for fluorescing dipole-dipole interacting atoms

A simplified scheme for the investigation of cooperative effects in the quantum jump statistics of small numbers of fluorescing atoms and ions in a trap is presented. It allows the analytic treatment of three dipole-dipole interacting four-level systems which model the relevant level scheme of Ba⁺ ions. For the latter, a huge rate of double and triple jumps was reported in a former experiment and the huge rate was attributed to the dipole-dipole interaction. Our theoretical results show that the effect of the dipole-dipole interaction on these rates is at most 5% and that for the parameter values of the experiment there is practically no effect. Consequently it seems that the dipole-dipole interaction can be ruled out as a possible explanation for the huge rates reported in the experiment.     

Measurement master equation

We derive a master equation describing the evolution of a quantum system subjected to a sequence of observations. These measurements occur randomly at a given rate and can be of a very general form. As an example, we analyse the effects of these measurements on the evolution of a two-level atom driven by an electromagnetic field. For the associated quantum trajectories we find Rabi oscillations, Zeno-effect type behaviour and random telegraph evolution spawned by mini quantum jumps as we change the rates and strengths of measurement.     

Coherent coupling of a single <Superscript>40</Superscript>Ca<Superscript>+</Superscript> ion to a high-finesse optical cavity

We demonstrate coherent coupling of the quadrupole S_1/2D_5/2 optical transition of a single trapped ⁴⁰Ca⁺ ion to the standing wave field of a high-finesse cavity. The dependence of the coupling on temporal dynamics and spatial variations of the intracavity field is investigated in detail. By precisely controlling the position of the ion in the cavity standing wave field and by selectively exciting vibrational state-changing transitions the ion’s quantized vibration in the trap is deterministically coupled to the cavity mode. We confirm coherent interaction of ion and cavity field by exciting Rabi oscillations with short resonant laser pulses injected into the cavity, which is frequency-stabilized to the atomic transition. Received: 23 August 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. E-mail: christoph.becher@uibk.ac.at RID="**" ID="**"Present address: Time and Frequency Division, National Institute of Standards and Technology, Boulder, CO 80305, USA     

Six-Fold Degenerate Dark States in a Four-Level Atomic System

With all driving fields on Raman resonance, a tripod-type atomic system quickly evolves into a dark state decoupled from the lossy excited level. The dark state depends strongly on field Rabi frequencies, spontaneous decay rates, and the initial atomic population in a complicated way. Analytical results reveal that it is a sixfold degenerate dark state with its three components superposed both coherently and incoherently due to population redistribution from spontaneous emission.     

Spin-polarized tunneling in ferromagnetic double barrier junctions

Spin-polarized tunneling in FMS/M/FMS double tunnel junctions where FMSs are ferromagnetic semiconductor layers and M is a metal spacer is studied theoretically within the single-site coherent potential approximation (CPA). The exchange interaction between a conduction electron and localized moment of the magnetic ion is treated in the framework of the s-f model. The spin polarization in the FMS layers is observed to oscillates as a function of the number of atomic planes in the spacer layer. Amplitude of these oscillations decreases with increasing the exchange interaction in FMS layers. Received 9 June 2001 and Received in final form 20 August 2001     

Isotope selective loading of an ion trap using resonance-enhanced two-photon ionization

We have demonstrated that resonance-enhanced two-photon ionization of atomic beams provides an effective tool for isotope selective loading of ions into a linear Paul trap. Using a tunable, narrow-bandwidth, continuous wave (cw) laser system for the ionization process, we have succeeded in producing Mg⁺ and Ca⁺ ions at rates controlled by the atomic beam flux, the laser intensity, and the laser frequency detuning from resonance. We have observed that with a proper choice of control parameters, it is rather easy to load a specific number of ions into a string. This observation has direct applications in quantum optics and quantum computation experiments. Furthermore, resonant photo-ionization loading facilitates the formation of large isotope-pure Coulomb crystals. Received: 21 December 1999 / Published online: 11 May 2000     

Spontaneous emission from a Λ three-level atom in an anisotropic photonic crystal

The spontaneous emission properties of a Λ type atom embedded in a three-dimensional anisotropic photonic crystal are investigated. Only one of the two atomic transition frequencies is considered near the photonic band edge. The atomic decay properties such as the time-evolution of the excited-state population and the instant and effective decay rates are studied in detail. It is found that there exists a wide region for the difference of the transition frequency from the band edge, in which only diffusion fields with frequencies being near or far away from the band edge appear in the emitted field. The spontaneous emitted field and its spectrum depend not only on the detuning of the transition frequency from the band edge, but also on the distance from the atom. Therefore, during the propagating process, the propagating field is partially transferred into the diffusion field.     

Solute effects on the helix-coil transition

We discuss the effects of the solvent composition on the helix-coil transition of a polypeptide chain. We use a simple model to demonstrate that improving the hydrogen-bonding ability of the solvent can make the transition less cooperative, without affecting the transition temperature. This effect is very different from other solvent effects which primarily influence the melting transition rather than the cooperativity. Received 10 December 2001 and Received in final form 22 March 2002     

Dark periods in the resonance fluorescence of a single Λ system

We use the quantum jump method to study the photon statistics of a single laser-driven atom in the configuration where both lower levels are strongly coupled to the common upper level. Under certain conditions we show that, for almost degenerate lower levels, light and dark periods occur which are similar to those of the well-known Dehmelt V system. Analytic results for their mean lengths and other statistical properties are given. For large separation of the lower levels we prove an interesting bunching property by the photons in the resonance fluorescence near the dark resonance. We propose a realistic system for which these effects may be observed.     

Adiabatic focusing of cold atoms in a blue-detuned laser standing wave

Adiabatic focusing of cold atoms in a blue-detuned laser standing wave is analyzed. It is shown that using repulsive light forces that push atoms towards dark regions and thus minimizes heating, cold atoms can be adiabatically compressed by more than an order of magnitude to yield background-free sub-10-nm (rms) spots. The optimal parameters for the atomic lens and the maximal compression ratio are predicted using an analytic model and found to be in agreement with the exact results of our Monte Carlo simulations. A combined adiabatic-coherent scheme is proposed and shown to yield 8.8 nm spot size even for a thermal atomic beam. Received: 1 October 1999 / Revised version: 13 December 1999 / Published online: 5 April 2000     

An optical conveyor belt for single neutral atoms

Using optical dipole forces we have realized controlled transport of a single or any desired small number of neutral atoms over a distance of a centimeter with sub-micrometer precision. A standing wave dipole trap is loaded with a prescribed number of cesium atoms from a magneto-optical trap. Mutual detuning of the counter-propagating laser beams moves the interference pattern, allowing us to accelerate and stop the atoms at preselected points along the standing wave. The transportation efficiency is close to 100%. This optical ‘single-atom conveyor belt’ represents a versatile tool for future experiments requiring deterministic delivery of a prescribed number of atoms on demand. Received: 6 July 2001 / Published online: 23 November 2001     

Collective atomic spin squeezing and control

Using a quantum theory for an ensemble of two- or three-level atoms driven by electromagnetic fields in an optical cavity, we show that the various spins associated with the atomic ensemble can be squeezed. Two kinds of squeezing are obtained: on the one hand self-spin squeezing when the input fields are coherent ones and the atomic ensemble exhibits a large non-linearity; on the other hand squeezing transfer when one of the incoming fields is squeezed. Received 14 August 2001 and Received in final form 7 November 2001     

Colored noise in a two-dimensional nonlinear system

A two-dimensional decoupling theory is developed when colored noise is included in a nonlinear dynamical system. By a functional analysis, the colored noise is transformed to an effective noise that includes the noise correlation time, the mean dynamical variable, and the original noise strength. When the two-dimensional decoupling theory is applied to single-mode and two-mode dye laser systems, the mean, variance, and effective eigenvalue of laser intensity are calculated. Excellent agreement between theoretical analysis, numerical simulations, and experimental measurements are obtained. It is seen that the increase of noise correlation time can reduce the fluctuations in the laser system. It is also shown that there is relatively large fluctuation in the phase when the laser undergoes from thermal light to coherent light when the theory is applied to a single mode dye laser. Received 20 August 2001 and Received in final form 4 December 2001     

Laser cooling with electromagnetically induced transparency: application to trapped samples of ions or neutral atoms

A novel method of ground-state laser cooling of trapped atoms utilizes the absorption profile of a three- (or multi-) level system that is tailored by a quantum interference. With cooling rates comparable to conventional sideband cooling, lower final temperatures may be achieved. The method was experimentally implemented to cool a single Ca⁺ ion to its vibrational ground state. Since a broad band of vibrational frequencies can be cooled simultaneously, the technique will be particularly useful for the cooling of larger ion strings, thereby being of great practical importance for initializing a quantum register based on trapped ions. We also discuss its application to different level schemes and for ground-state cooling of neutral atoms trapped by a far-detuned standing wave laser field. Received: 10 July 2001 / Published online: 23 November 2001     

Near-field effect in two-atom system

The self-consistent problem is solved for the interaction of two dipole atoms situated at arbitrary distance from one another with the field of quasiresonant light wave. Atoms are considered to be linear Lorenz oscillators. Polarizing fields inside the system include both Coulomb and retarding parts. The solutions obtained are investigated for the case when atoms have the same polarizabilities and interatomic distance is much less than external light wavelength. Formulas for electric fields inside and outside of small object are obtained. It is shown that longitudinal and transverse optical oscillations are possible to exist inside small two-atom object. Dispersion laws of these oscillations depend upon interatomic distance and upon angle between axis of the system and the direction of propagation of external wave. The field outside the small object in wave zone is linearly polarized with the choice of linear polarization of external field. However, the directions of polarization of these waves are different and depend essentially upon frequency. The amplitude of field outside small object in wave zone is shown to depend essentially on the frequency of external field and interatomic distance. The results obtained are treated as near-field effect in the optics of small objects making it possible to investigate the structure of small objects with optical radiation. Received 26 October 1998 and Received in final form 26 January 2000     

Observation of collimation and decollimation of an atomic beam in a misaligned standing wave

This paper reports an experimental study on the collimation and decollimation of an atomic beam in a misaligned standing wave, in which the effective detuning caused by the Doppler effect is affected by the longitudinal velocity of the atomic beam. The experiment shows that in a strong field with red detuning between laser field and atomic transition frequency, laser heating in a normal standing wave becomes laser cooling in a misaligned standing wave for an approriate misalignment angle. For blue detuning, laser cooling in a standing wave can also become laser heating in a misaligned standing wave for an appropriate condition. These results ca be used in controling atomic motion.