Observation of laser-cooled Be+ -ion clouds in a Penning trap

Be⁺ ions trapped in a Penning trap are laser-cooled to about 10 mK. The excitation spectra of ion clouds containing about 500 ions are obtained by scanning the frequency of the cooling laser and discontinuities in these spectra are observed because of phase transitions. When the cooled ions are heated electrically by applying an rf voltage, no phase transition occurs and the spectra become continuous. Two-dimensional measurement of the ion clouds is carried out and the abrupt change in the shape of the ion cloud due to the phase transition is observed. When many ions are trapped and cooled, the phase transition occurs partially and a transient state where two states are mixed can be observed. The static properties of the ions are also measured by using an additional probe laser and the results of experimental measurements are compared with theoretical predictions.     

Investigation of sub-Doppler cooling effects in a cesium magneto-optical trap

We present an investigation of sub-Doppler effects in a cesium magneto-optical trap. First, a simple one-dimensional theoretical model of the trap is developed for aJ _g = 1 J _e = 2 transition. This model predicts the size of the trapped atom cloud and temperature as a function of laser intensity and detuning. In the limit of small magnetic field gradients, the trap temperature is found to be equal to the molasses temperature and a minimum size for the trap is calculated. We then describe several experiments performed in a three-dimensional cesium trap to measure the trap parameters, spring constant, friction coefficient, temperature and density. Whilst the temperature of the trapped atoms is found to be equal to the molasses temperature, in agreement with theory, the trap spring constant is found to be two orders of magnitude smaller than the one-dimensional prediction, a value close to that predicted by Doppler models. The maximum density is found to be on the order of 10¹² atoms/cm³ or one atom per optical wavelength on average. When the number of trapped atoms becomes large, the temperature begins to increase dramatically. This excess temperature depends in a very simple way on the atom number, laser intensity and detuning, suggesting that its origin lies in multiple photon scattering within the trap.     

Observations of anomalous “quantum jumps” in a single ion trap

Cooled positive barium ions, proposed for new frequency standards, cease scattering cooling radiation for anomalously long periods in the presence of carbon dioxide or water. The probable mechanism is the attraction of a gas molecule to the ion as a result of the dipole induced in the molecule by the ion, followed by the formation of a weak chemical bond between the two. The mechanism seems applicable to any ion.     

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     

Potential usage of the magnetron-motion-free mode of one ion confined in a combined trap

Received: 5 February 1998/Revised version: 18 March 1998     

Sympathetic cooling rate of gas-phase ions in a radio-frequency-quadrupole ion trap

We theoretically investigated the mass dependence of the sympathetic cooling rate of gas-phase ions trapped in a linear radio-frequency-quadrupole ion trap. Using an a priori molecular dynamical calculation, tracing numerically with Newtonian equations of motion, we found that ions with a mass greater than 0.54±0.04 times that of the laser-cooled ions are sympathetically cooled; otherwise, they are heated. To understand the mass dependence obtained using the molecular-dynamical calculation, we made a heat-exchange model of sympathetic cooling, which shows that the factor of 0.54±0.04 is a consequence of absence of micro-motion along the axis of the linear ion trap. Received: 10 December 2001 / Revised version: 28 January 2002 / Published online: 14 March 2002     

Quantum dynamics of bouncing atoms in a stable gravitational cavity

Two-level atoms bouncing in a stable gravitational cavity are considered, where the atomic mirror at the bottom of the bounces is an evanescent wave caused by an internally reflected intense Gaussian-mode laser beam. We consider the broadening mechanisms of the atoms from their initially tightly spaced position distribution, using a phenomenological semi-classical model, which includes spontaneous emission. A fully quantum model, which neglects spontaneous emission, is derived, and the broadening of the atomic wave function in the quantum model is compared with the broadening of the atomic distribution in an analogous classical simulation where spontaneous emission is similarly neglected. We find that the broadening is correctly described by the classical simulations in the horizontal directions, while it significantly underestimates the broadening in the vertical direction.Dedicated to H. Walther on the occasion of his 60th birthday     

Atomic beam guide by a one-dimensional magneto-optical trap

An atomic beam has been collimated, compressed, and deflected simultaneously by an atomic beam guide based on an inclined one-dimensional magneto-optical trap (1D-MOT). Isotope-selected rubidium atoms were extracted from the naturally-mixed thermal atomic beam with this method. We could manipulate the transverse displacement of the deflected beam precisely by adjusting the current in the copper rods to generate the quadrupole magnetic field. We could extract more than 50% of the incident atoms as a deflection beam when we combined this deflection technique with the atomic deceleration using a broadband spectral light. Received: 10 December 1998 / Published online: 24 June 1999     

Quantized infrared-optical triple resonance on a single cold barium ion

A single trapped and cooled Ba⁺ ion is irradiated by resonant visible light (493, 650 nm) alternating with light at 1.76 µm which may excite the ion to its² D _5/2 metastable state. The (absence of) visible resonance scattering probes the excitation, tuning spectra of which show vibrational sidebands that characterize the ion's temperature. Observed values as low as 120 µK, one-eighth the Doppler limit, are ascribed to electronic Raman cooling by the visible light. Tuning spectra of the events of stimulated deexcitation indicate ion heating by the IR interaction. The results demonstrate the feasibility of vibrational spectrometry on a single particle that oscillates in a potential well, forming a quasi-molecule.     

Self-induced zeeman coherences in a Paul trap

A new method for the measurement of motional frequencies and amplitudes of stored ions in a radio-frequency trap is presented. Ions oscillating in the trap potential and additionally subjected to a small magnetic field, undergo sublevel transitions between adjacent Zeeman states when their motional frequency is identical with the Larmor frequency in the applied magnetic field. These transitions can be sensitively detected by means of an optical pumping scheme. As they are related to a coherent superposition of adjacent states and originate from the inherent motion of the ions in a slightly inhomogeneous magnetic field, this phenomenon is termed self-induced Zeeman coherence.     

Observation of Doppler sidebands of a laser-cooled Ca+ ion by using a low-temperature-operated laser diode

1/2 -D_5/2 electric-quadrupole-allowed transitions of a laser-cooled Ca⁺ ion in a small rf trap. The electron shelving method was used to measure the absorption spectrum of the electric-quadrupole-allowed transitions, and the motional sidebands due to the secular motion of the ion in the harmonic potential well of the rf trap were completely resolved. The effective temperature of the ion, estimated by comparing the observed sideband intensities with the theoretical ones, was less than 4.4 mK. This result is in good agreement with that obtained from the analysis of the linewidth measurement. Received: 18 March 1998     

The ion trap quantum information processor

Received: 16 October 1996/Revised version: 2 April 1997     

Decay rate measurement of lithium in a magneto-optical trap

A detailed account of various experimental techniques developed during the study on the decay rate coefficient of laser trapped ⁷Li atoms are presented. The frequency of a dye laser is stabilized using a simple sealed-off cell specially designed for Li vapor. The accurate number of trapped atoms are obtained by measuring the fluorescence intensity and the population ratio between the ground and the excited states by absorption coefficient measurement. The absolute value of the collisional lossrate coefficient of trapped ⁷Li atoms is determined by analyzing the temporal change of the fluorescence intensity when the supply of the Li beam is turned off.     

Isotope separation by nonlinear resonances in a Paul trap

Deviations from the ideal quadrupole potential in a Paul ion trap create nonlinear resonances at certain operating points inside the stability diagram, where in the absence of potential pertubations storing times are very long. In the presence of those pertubations, however, the ions are lost from the trap. Since these resonances are mass dependent and the mass resolution is of the order of 100 it can be used to separate isotopes of a given element by choosing suitable trap operating conditions. Experiments on a natural mixture of Eu⁺ ions of mass 151 and 153 show that in a simple way, by proper choice of the operating point, the ions can be completely separated and laser-induced optical spectra of a single isotope can be received. This is the first time that mass separation in a Paul trap is performed by nonlinear effects in contrast to the usual way of using the mass dependent boundaries of the stability diagram     

Separation of laser-cooled 42Ca+ and 44Ca+ in a linear Paul trap

Rare calcium isotope ions, ⁴²Ca⁺ and ⁴⁴Ca⁺, were efficiently separated from a laser-cooled ionic cloud in a linear Paul trap which was loaded from an atomic source of a natural isotope mixture of calcium, in spite of their negligibly small abundances (0.647% and 2.086%, respectively) compared with that of ⁴⁰Ca (96.94%). Selective heating and cooling that arise from the isotope shifts were mainly used for the elimination of the isotopes; selective heating and cooling enable flexible separation when combined with other mechanisms such as the inherent mass selectivity of RF traps. Received: 7 July 2000 / Published online: 10 January 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     

Laser cooling of a small number of Be+ ions in a penning trap

Be⁺ ions stored in a Penning trap were cooled by a laser beam perpendicular to the magnetic field. The cooled ions are strongly coupled and phase transitions of up to 100 ions were observed. In experiments with only a few ions stored in the trap, a stepwise decrease in fluorescence intensity was observed. All steps are of the same size and so every step is attributed to a single ion. The discrete changes in fluorescence occurred more frequently when the background pressure was increased, caused by collisions between stored ions and background neutral molecules.     

Dark resonances as a probe for the motional state of a single ion

Single, rf-trapped ions find various applications ranging from metrology to quantum computation. High-resolution interrogation of an extremely weak transition under best observation conditions requires an ion almost at rest. To avoid line-broadening effects such as the second-order Doppler effect or rf heating in the absence of laser cooling, excess micromotion has to be eliminated as far as possible. In this paper the motional state of a confined three-level ion is probed, taking advantage of the high sensitivity of observed dark resonances to the trapped ion’s velocity. Excess micromotion is controlled by monitoring the dark-resonance contrast with varying laser-beam geometry. The influence of different parameters such as the cooling laser intensity has been investigated experimentally and numerically.     

Kinetic energy and spatial width of ion clouds in paul traps

The ratio of the mean kinetic energy and the spatial width of ion clouds in Paul traps has been measured. The result is compared with predictions of various models that describe ion cloud dynamics in Paul traps.     

Trapping atoms in a gravitational cavity

This paper is devoted to the study of a new atomic cavity consisting of a single horizontal concave mirror placed in the earth gravitational field. Gravity, by bending the atomic trajectories, plays the role of a second mirror closing the cavity. We first discuss the stability criterion for this cavity, assuming that the mirror has a parabolic shape. We then derive the quantum mechanical modes of such a configuration, with particular emphasis on the paraxial (i.e., close to vertical) motion. Finally, we discuss the possibility of populating those modes from an initial cold atomic cloud dropped above the mirror.Laboratoire associé au CNRS et à l'Université Pierre et Marie Curie