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Physica Scripta

Quantum optics with trapped and laser cooled magnesium ions

Richard Claude Thompson, David John Bate, Kishan Dholakia, D M Segal, D. C. Wilson

Published Sep 1, 1992• 13 citations
Publisher

Abstract

We report recent experiments at Imperial College with magnesium ions confined in an ion trap and cooled by irradiation with a frequency-doubled dye laser. The work covers observation of quantum jumps in single ions in a Penning trap, measurement of oscillation frequencies in the combined trap, and a preliminary study of the spectrum of time intervals between consecutive detected photons. Theoretical considerations relevant to the experiments are also presented.

DOI
10.1088/0031-8949/46/3/014

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We report recent experiments at Imperial College with magnesium ions confined in an ion trap and cooled by irradiation with a frequency-doubled dye laser. The work covers observation of quantum jumps in single ions in a Penning trap, measurement of oscillation frequencies in the combined trap, and a preliminary study of the spectrum of time intervals between consecutive detected photons. Theoretical considerations relevant to the experiments are also presented.
We have recently constructed an ion trapping and laser cooling system at Imperial College suitable for magnesium ions. This system consists of a Penning ion trap contained within an all-metal vacuum system maintained below 3 × 10−10 mbar. Also enclosed in the vacuum system are small magnesium beam ovens made of tantalum foil and resistively heated by a current of typically 6 A. The ring electrode (see Fig. 1) is kept at a negative potential with respect to the endcaps, and the magnetic field of ~1 T perpendicular to the plane of the ring causes the ions to move in closed orbits within the trap.
This thesis describes experimental work performed on laser cooled magnesium ions, confined in a quadrupole ion trap operating in the Penning or the combined mode. Information is gained about the dynamical motion of the ions. This is done by analyzing the time delays between scattered fluorescence photons in the case of small ion clouds (generally less than ten ions). For larger clouds (upwards of 100 ions), a fraction of the ions is "tagged" and a subsequent study made of the time evolution of the fluorescence. We have developed an innovative, non-invasive method for measuring ion oscillation frequencies which is based on analyzing photon-photon correlations present in the emitted ion fluorescence. This technique has been used to measure all three Penning trap frequencies for a wide range of dc voltage. The data have been fitted to theory and we have obtained a global fit of these data to one set of trap parameters. We also demonstrate that our technique enables us to measure space-charge shifted values of the trap resonances. This method has also been applied to the combined trap and we report experimental verification of the predicted trap resonances for a wide range of trapping parameters. These data fit well to theory. This is for all three oscillation frequencies for one set of trap parameters consistent with the data for the Penning trap. \nFurther experiments are reported concerned with the dynamics of larger ion clouds in a Penning trap. A pulsed laser is used to "tag" a small number of the ions in large ion clouds. This removes those ions from the cooling cycle by placing them in a metastable state. Subsequently, we have monitored the time evolution of the fluorescence. By analyzing these data we have been able to determine cloud rotation frequencies for large ensembles of ions.
Summary form only given. We have performed a study of quantum jumps in single magnesium ions held in a Penning trap. This system is unusual in that quantum jumps may be observed using only a single laser frequency which serves to laser cool the ion (providing fluorescence for detection), shelve the ion in a metastable state (via an off-resonant Raman transition) and also to repump the ion from the metastable state back into the cooling cycle (again via a Raman transition). Single /sup 24/Mg/sup +/ ions have been studied in a Penning trap before but we have predicted and now observed a number of interesting effects that occur only in /sup 25/Mg/sup +/. We have also trapped single ions of /sup 26/Mg/sup +/ and confirmed that their quantum jump behaviour is similar to that of /sup 24/Mg/sup +/ as expected. The novel features in the behaviour of /sup 25/Mg/sup +/ are due to the presence of hyperfine structure in this ion.
This paper is a written version of an invited review talk given at the 11th National Quantum Electronics Conference in Belfast in September 1993. It discusses the use of trapped ions for spectroscopy in both the optical and microwave regions of the spectrum. After introductory comments on the advantages offered by ion traps and laser cooling for spectroscopy, we take the the example of spectroscopy of mercury ions to show how many different types of experiment can be performed, with striking results. We also discuss our recent work at Imperial College, which can be seen as spectroscopy of the ion trap itself, giving information on the dynamics of the trapped ions.
We have performed laser cooling of Mg+{}^{+}+ ions confined in a Penning trap. The externally produced ions were captured in flight, stored, and laser cooled. Laser-induced fluorescence was observed perpendicular to the cooling laser axis. Optical detection down to the single-ion level together with the electronic detection of the ion oscillations inside the Penning trap have been used to acquire information on the ion storage time, ion number, and ion temperature. Evidence for the formation of ion crystals has been observed. These investigations are an important prerequisite for the sympathetic cooling of simultaneously stored highly charged ions and the precision laser spectroscopy of forbidden transitions in these.