Th. Sauter

‘Quantum jumps’ observed in the fluorescence of a single ion

Abstract The laser-excited resonance fluorescence of a single Ba + ion at 493 nm shows interruptions of macroscopic time durations when the ion drops into a dark state by off-resonant Raman scattering, or by weak excitation of the 2 P 3/2 level. We manipulate the ion out of the state by auxiliary laser light and prove that state to be the metastable 2 D 5/2 level. Fluorescence from three ions displays an enhanced rate of multiple “jumps” which reveals cooperative ionic interaction with the light.

On the photo-dynamics of single ions in a trap

Early this century, atomic beams were the closest approach tofree atomic particles. Nowadays, individual ions are stored in traps for, in principle, unlimited times of experimentation. The recoil of absorbed or scattered light permits us to manipulate the ion kinetics: Cooling and heating the ion reveals novel types of nonlinear light forces, one of which results from an all-stimulated parametric process similar to the action of a free-electron laser. — The amplitude of the vibration of a single ion in the potential well of the trap locks to metastable levels of excitation. Few ions form crystals or rotating quasi-molecules. Random exchange of sites is demonstrated by a quasi-molecule composed of a fluorescing and a dark ion.

‘‘Quantum jumps’’ observed in the fluorescence of a single ion

We demonstrated interruptions of macroscopic duration in a single trapped and cooled Ba+ ions’s 493‐nm fluorescence. They are caused by transitions of the ion into the ‘‘dark’’ 2D5/2 state.—Multiple simultaneous jumps of three ions indicate cooperative interaction with the light.

Quantum Jumps in a Single Ion

The laser-excited resonance fluorescence of a single atomic particle is interrupted for macroscopic time intervals, while the particle, after having undergone a quantum jump,1 resides in a metastable (or OFF) state. Therefore, these dark intervals serve as the signature of quantum jumps to the OFF state and back to the ON state. For a Ba+ ion interacting with laser light at 493 and 650 nm,2,3 the ON state is a superimposition of the 2S1/2, 2Р1/2, and 2D3/2 levels.

The Kinetic Energy of Ion Clouds in Paul Traps

The dynamics of ion clouds in Paul traps has been experimentally investigated. Kinetic energy and the width of the spatial distribution of ions has been measured. Their dependence on the traps operating conditions is compared with the results of a recent model calculation based on Brownian motion.

“Quantum jumps” observed in single-ion fluorescence

We have demonstrated that interruptions of macroscopic duration appear in the 493-nm fluorescence of a single trapped and cooled Ba+ ion. They are caused by sudden transitions of the ion into the “dark” 2D5/2 state. — Multiple simultaneous jumps of three ions indicate cooperative interaction with the light.

Coherence of states in trapped ions

The formation of Hertzian coherence of equal-parity states in trapped ions by the application of double-resonance interaction schemes has produced ultra-narrow resonances with quality factors on the order of 1011. Optical coherences of equal-parity states may be responsible for the identification of finer details of the excitation spectrum of single trapped ions at weel as for some novel contributions to their mechanical interaction with light. These effects seem tob e undetectable in gases since collisions are present adn the interaction time is limited.

Nonlinear Effects in Trapped Three-Level Ions

Single trapped and laser-cooled ions have been considered so far paradigms of the ultimate high-resolution spectroscopy and photon statistics. They also represent individual systems with coupled internal and external degrees of freedom simple enough to allow detailed comparison of their kinetics with current models. — We have observed several metastable vibrational states of a single laser-illuminated Ba ion which become excited upon gently heating the ion. They correspond to linear oscillatory modes in r, z directions, which have been documented by image conversion and video recording. — Suitable normalization of the photon-counted and spatially resolved signals of resonance fluorescence allows us to discriminate the features of the electronic excitation spectra and results in spectra of the ion’s vibrational excitation. These spectra reveal two novel types of light force which we ascribe to stimulated two-photon transitions followed by Raman scattering, and to all-stimulated parametric action. — These observations bear upon the interpretation of recent observations on the kinetics of atoms confined in optical molasses.

Kinetic Energy and Spatial Distribution of Ion Clouds in Paul Traps

Ion traps provide a very favourable environment for the conduction of experiments aimed at high spectral resolution: They allow the preparation of ionic samples localized in space free of collisions with walls [1]. In this situation, the second order Doppler effect is the largest systematic error, which can be reduced by laser cooling of the stored ions [2]. However, this merely works with few ions and yields only low signal-to-noise ratio. Another approach is to measure or to calculate the second-order Doppler effect and to hold it constant. In order to do so, an investigation of the dynamics of ion clouds has been performed. The ions’ motion is described as Brownian motion in the time-dependent trapping potential. Both damping and diffusion terms due to collisions with the background gas and other ions are introduced on a phenomenological basis. The corresponding Fokker-Planck-equation has been solved analytically and results in the joint distribution of position and velocity as a function of the trap parameters. This Gaussian distribution has time-dependent variances and shows a correlation between position and velocity. With that, the mean kinetic energy can be calculated and a correction for the second-order Doppler effect can be extracted [3].

Quantum Jumps and Related Phenomena of Single Ions and Small Ion Clouds

Experimentation with single “live” atomic particles — ions — has turned real a few years ago /1/. Precondition for this achievement was the development of techniques for storage and detection of small ion clouds, and in particular for damping the ion motion in the trap: optical cooling was predicted for free /2/ and bound particles /3/ in 19 75 and subsequently observed /4,5/. This development has been crucial for an attempt to get evidence of and insight into a concept which is intrinsic to BOHR’s successful model of the hydrogen atom /6/: instantaneous transitions between energy eigenstates of an atom upon interaction with light, which have been labelled “quantum jumps”. The early controversy on that, as it seemed, awkward concept subsided: quantum mechanics turned out a story of overwhelming success after all, and repeatable experimenting with single atoms was considered unthinkable, anyway. As for traces in bubble chambers and on photographic plates, Erwin SCHRODINGER rightfully cautioned: “In the first place it is fair to state that we are not experimenting with single particles , any more than we can raise Ichthyosauria in the zoo, We are scrutinising records of events long after they have happened… ” /7/.