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Isotopic effects on quantum jumps in trapped magnesium ions

David John Bate, Richard Claude Thompson, D. C. Wilson

Published May 21, 1990• 0 citations
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Abstract

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.

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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.

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Journal of Physics B Atomic Molecular and Optical Physics•Jul 15, 2009•20 citations
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.
The quantum jump statistics for the three stable isotopes of Mg + have been studied experimentally. Single ions of magnesium were held and laser cooled in a Penning trap. Quantum jumps were observed by collecting the resonance fluorescence near 280 nm from the cooled ion. The jump rate was analysed to find ratios of the average time the ion spends fluorescing, `on' time, to the average time it spends in the dark state, `off' time: for both 24 Mg + and 26 Mg + this value was found to be 15.9±1.2, in agreement with a theoretical value of 16.0. The presence of hyperfine structure in 25 Mg + means that while the jump rate for 24 Mg + and 26 Mg + remains unaltered for both available cooling transitions, for the odd isotope the on to off ratio is predicted to be 11.8 and 22.8 for the lower and upper transitions respectively. Experimental values for these ratios were found to be 10.2 ± 0.7 and 22.0 ± 0.6. The occurrence of so-called nuclear jumps in 25 Mg + is investigated qualitatively. These rare jumps are produced as a result of the hyperfine splitting, which is only present in the odd isotope. Although the data indicate some evidence for this type of jump, it is not possible to draw any firm conclusions due to the small number of events observed.
The long term aim of this work is to study the suitability of using laser cooled Ca+ ions in Penning traps as the basic components of a quantum computer. A great deal of progress in the field of quantum computing has been made in recent years with laser cooled ions stored in radio frequency ion traps. Building a useful quantum computer with trapped ions is however extremely challenging. Penning traps offer some possible benefits over radio frequency traps. They also create some additional difficulties. The potential advantages and disadvantages of Penning traps are discussed throughout the thesis. We show that we are able to overcome the problems associated with laser cooling in Penning traps, and have trapped single ions for extended periods of time. Pairs of Ca+ ions have been aligned along the axis of a Penning trap, and have been optically resolved. A novel Penning trap array based on PCB boards has been developed. A prototype was built and tested, along with the electronics required to shuttle ions between different sub-traps. Ions have been shuttled a distance of 10 mm in 2.5 μs. A return trip efficiency of up to 75% was seen. A quantum effect – J-state mixing caused by large magnetic fields – has been observed for the first time in single atomic ions. The magnetic field causes a forbidden [Delta]J = 2 transition to become weakly allowed. This effect is of general interest in atomic physics, and is also very relevant for quantum computation studies. A quantitative prediction of the magnitude of the J-mixing effect has been derived theoretically. This is compared to experimental data, and is found to be in excellent qualitative and good quantitative agreement.
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.