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     

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     

Five-beam magneto-optical trap and optical molasses

We have operated a magneto-optical trap and optical molasses for the laser cooling of cesium atoms on the basis of a five-beam laser configuration. For the magneto-optical trap two laser beams counterpropagate along the axis of a quadrupole trap and the remaining three beams propagate in the orthogonal plane at 120° to each other. The same optical configuration was used for the optical molasses. We have tested the efficiency in atom collection and the temperatures reached in both cooling processes. In comparison to previous results on a six-beam configuration, a lower number of atoms is collected, while comparable densities are realized. The atomic temperatures have been measured through a delayed shadow-image technique, where one of the running-wave cooling beams produces an absorptive image of the atoms on a camera. Received: 14 January 1999 / revised version: 23 June 1999 / Published online: 8 September 1999     

Decoding the dynamics of a single trapped atom from photon correlations

Received: 4 September 1998     

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     

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     

One atom in an optical cavity: Spatial resolution beyond the standard diffraction limit

The position of a slow atom passing through a standing-wave light field in an ultrahigh-finesse optical resonator can be measured by observing either the intensity of the light transmitted through the cavity or its phase. Apart from the periodicity of the standing wave, both techniques allow to determine the position of the particle with a resolution much better than the standard classical diffraction limit /2. Position measurements with uncertainty </20 seem to be possible with all-optical techniques.These notes were prepared to celebrate H. Walther's 60th birthday and to honour his pioneering contributions to some of the most lively fields of quantum optics     

Laser cooling in a CO2-laser optical lattice

Received: 19 June 1998     

Quantum transport of ultracold atoms in an accelerating optical potential

Received: 8 July 1997     

All-diode-laser cooling of single Ca+ ions

+ ions. The Ca⁺ ions are trapped in a miniature rf Paul trap and irradiated by light from a frequency-doubled diode laser at 397 nm and by light from a diode laser at 866 nm. We are able to cool a single ion and observe its fluorescence continuously with the laser diode locked to the external frequency-doubling cavity. Quantum jumps in the fluorescence light of a single ion and of a small cloud of five ions have been induced by driving the “clock” transition at 729 nm. We were able to resolve the influence of the micromotion on the excitation spectrum of the small ion cloud. Received: 10 July 1997/Revised version: 17 November 1997     

Controlled generation of single photons from a strongly coupled atom-cavity system

We propose a new method for the generation of single photons. Our scheme will lead to the emission of one photon into a single mode of the radiation field in response to a trigger event. This photon is emitted from an atom strongly coupled to a high-finesse optical cavity, and the trigger is a classical light pulse. The device combines cavity-QED with an adiabatic transfer technique. We simulate this process numerically and show that it is possible to control the temporal behaviour of the photon emission probability by the shape and the detuning of the trigger pulse. An extension of the scheme with a reloading mechanism will allow one to emit a bit-stream of photons at a given rate. Received: 7 July 1999 / Revised version: 3 September 1999 / Published online: 20 October 1999     

Collisional effects on the collective laser cooling . of trapped bosonic gases

We analyse the effects of atom–atom collisions on a collective laser cooling scheme. We derive a quantum master equation which describes the laser cooling in presence of atom–atom collisions in the weak-condensation regime. Using such equation, we perform Monte Carlo simulations of the population dynamics in one and three dimensions. We observe that the ground-state laser-induced condensation is maintained in the presence of collisions. Laser cooling causes a transition from a Bose–Einstein distribution describing collisionally induced equilibrium, to a distribution with an effective zero temperature. We analyse also the effects of atom–atom collisions on the cooling into an excited state of the trap. Received: 18 June 1999 / Revised version: 24 September 1999 / Published online: 10 November 1999     

Writing a superlattice with light forces

In atom lithography the conventional roles played by light and matter are reversed. Instead of using a solid mask to pattern a light beam, a mask of light is used to pattern a beam of neutral atoms. In this paper we report the production of different chromium dot arrays with quadratic symmetry. The lattice period depends on the relative polarization and the phase of the two standing waves generating the light mask. A small angular misalignment of the laser beams breaks the high symmetry and a chromium superlattice is written, that is a continuous periodic change between two different quadratic lattices. The structures exhibit lines with a FWHM below 50 nm and clearly separated chromium dots with a FWHM below 70 nm. Received: 30 September 1999 / Revised version: 14 February 2000 / Published online: 5 April 2000     

Quantum interferences in coherent population trapping of a cold double Λ-type atom

We investigate quantum interferences in coherent population trapping of a cold double Λ-type four-level atomic system driven by two counterpropagating laser fields. We study both decoherence and enhanced-coherence actions resulting from the multi-transition pathways in building up the trapping state, and analyze the system operating with and without external coherences in various configurations of the atomic dipole moments.     

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     

Ion strings for quantum gates

Received: 6 August 1997/Revised version: 21 October 1997     

Laser cooling of atoms in optical lattices including quantum statistics and dipole–dipole interactions

We develop a mean-field approach to include dipole-dipole interactions and quantum statistics in the atomic dynamics in bright and dark optical lattices including the proper spatial potentials instead of a simple δ-approximation. For classical distinguishable particles the results are even quantitatively similar to the properly scaled δ-function model. As the dominant effect bright and dark lattices exhibit opposite shifts in the lattice band energies and differ in their localisation properties as a function of density. The spatial-dependent potential allows strong modifications also in dark lattices, but the main conclusions obtained in the δ-approximation turn out to be still valid. Interestingly, important quantitative differences from the δ-model can occur as far as the effect of statistics in concerned, especially for fermions. We study the quantum statistical effects as a function of detuning and lattice well depths and identify the case of lattices with deep wells and large detunings as the preferred parameter region to observe them. Received 24 November 1999 / revised version: 24 June 1999 / Published online: 8 September 1999     

Dynamics of a two-level trapped ion in a cavity

In the present paper we consider the case of a two-level ion in a cavity in the presence of a single mode field linearly polarized. We suppose that the ion is free to move along the polarization direction and trapped by a harmonic potential along the other two directions. By multiple path integration we derive the density matrix of the system and we study its dynamics. We assume an initial electromagnetic vacuum. This initial condition for the present system, compared with any other initial photonic state, gives new and higher order leading terms with respect to an expansion in powers of the inverse of the volume. Further after such an expansion there appears a first order term that originates from the combined interaction of the two-level system (qubit) with the quantum motion of the ion and the electromagnetic field in the cavity. We notice that the dynamics of the present system is very rich and can be studied exhaustively in the present framework.     

Coherent excitation of Rydberg atoms in an ultracold gas

We demonstrate two schemes for the coherent excitation of Rydberg atoms in an ultracold gas of rubidium atoms employing the three-level ladder system 5S_1/2-5P_3/2-n?_j. In the first approach rapid adiabatic passage with pulsed laser fields yields Rydberg excitation probabilities of 90% in the center of the laser focus. In a second experiment two-photon Rydberg excitation with continuous-wave fields is applied which results in Rabi oscillations between the ground and Rydberg state. The experiments represent a prerequisite for the control of interactions in ultracold Rydberg gases and the application of ultracold Rydberg gases for quantum information processing.     

2D dark optical surface lattice with an efficient intensity-gradient cooling of atoms by evanescent wave interference

We propose a novel scheme to form a 2D dark optical surface lattice (DOSL) for cold atoms on the surface of the dense flint glass by using two sets of blue-detuned evanescent wave interference fields and a blue-detuned evanescent wave field. In the 2D DOSL, cold atoms will be trapped in the vicinity of minimum intensity and suffered the minimal light shift as well as the lowest coherence loss. The total potential and trap-depth of the individual optical micro-trap in the 2D DOSL are high enough to trap cold atoms (T = 120 μK) released from the standard magneto-optical trap (MOT), and atoms trapped in the 2D DOSL can be cooled to several μK with the efficient intensity-gradient Sisyphus cooling. The lattice constant of the DOSL can be controllable by changing the incident angles of lights.