Highly charged ions: Optical clocks and applications in fundamental physics

Abstract

Recent developments in frequency metrology and optical clocks have been based on electronic transitions in atoms and singly charged ions as references. The control over all relevant degrees of freedom in these atoms has enabled relative frequency uncertainties at a level of a few parts in 10 −18 . This accomplishment not only allows for extremely accurate time and frequency measurements, but also to probe our understanding of fundamental physics, such as a possible variation of fundamental constants, a violation of the local Lorentz invariance, and the existence of forces beyond the Standard Model of Physics. In addition, novel clocks are driving the development of sophisticated technical applications. Crucial for applications of clocks in fundamental physics are a high sensitivity to effects beyond the Standard Model and Einstein's Theory of Relativity and a small frequency uncertainty of the clock. Highly charged ions offer both. They have been proposed as highly accurate clocks, since they possess optical transitions which can be extremely narrow and less sensitive to external perturbations compared to current atomic clock species. The large selection of highly charged ions in different charge states offers narrow transitions that are among the most sensitive ones for a change in the finestructure constant and the electron-to-proton mass ratio, as well as other new physics effects. Recent experimental advances in trapping and sympathetic cooling of highly charged ions will in the future enable advanced quantum logic techniques for controlling motional and internal degrees of freedom and thus enable high accuracy optical spectroscopy. Theoretical progress in calculating the properties of selected highly charged ions has allowed the evaluation of systematic shifts and the prediction of the sensitivity to the "new physics" effects. New theoretical challenges and opportunities emerge from relativistic, quantum electrodynamics, and nuclear size contributions that become comparable with interelectronic correlations. This article reviews the current status of theory and experiment in the field, addresses specific electronic configurations and systems which show the most promising properties for research, their potential limitations, and the techniques for their study.

Robust optical clock transitions in trapped ions using dynamical decoupling

“…Optical clocks based on neutral atoms trapped in optical lattices and single trapped ions have reached estimated systematic uncertainties of a few parts in 10 −18 [1][2][3][4] or even below [5]. Taking advantage of these record uncertainties for applications ranging from relativistic geodesy [6][7][8][9] over fundamental physics [10][11][12] to frequency metrology [13][14][15][16][17] requires achieving statistical measurement uncertainties of the same level within practical averaging times τ (given in seconds). This has been achieved with single-ensemble optical lattice clocks in self-comparison experiments up to a level of 1.6 10 16 t - [18] and by implementing an effectively deadtime-free clock consisting of two independent clocks probed in an interleaved fashion [19,20], reaching a statistical uncertainty in the range of 5 10 17 t -.…”

Interactions and charge-transfer dynamics of anAl+ion immersed in ultracold Rb and Sr atoms

“…Trapped and laser-cooled atomic ions allow for tackling questions touching upon the very fundamentals of quantum mechanics [1]. They have found many applications ranging from quantum simulation and quantum computation [2,3] to quantum metrology and sensing [4][5][6]. For example, optical atomic clocks based on single trapped ions have been realized [7][8][9][10].…”

Accurate Prediction of Clock Transitions in a Highly Charged Ion with Complex Electronic Structure

“…Computational advances demonstrated in this work are widely applicable to most elements in the periodic table and will allow to solve numerous problems in atomic physics, astrophysics, and plasma physics.High resolution optical spectroscopy of highly charged ions (HCI) became a subject of much recent interest due to novel applications for the development of atomic clocks and search for new physics beyond the standard model of elementary particles [1][2][3][4]. HCI optical clock proposals, fundamental physics applications, and experimental progress towards HCI high-precision spectroscopy were recently reviewed in [4]. HCI have numerous optical transitions between long-lived states suitable for development of clocks with very low uncertainties, estimated to reach 10 −19 level [5][6][7][8].…”

“…HCI have numerous optical transitions between long-lived states suitable for development of clocks with very low uncertainties, estimated to reach 10 −19 level [5][6][7][8]. A particular attraction of HCI clock transitions is their exceptionally high sensitivity to a variation of the fine-structure constant α and, subsequently to dark matter searches [2][3][4].In many theories beyond the standard model, in particular those involving light scalar fields that naturally appear in cosmological models, the fundamental constants become dynamical (i.e. varying) [9][10][11][12][13][14].…”

“…Experimentally, variation of α is probed by monitoring the ratio of two clock frequencies with different values of K. Most of the currently operating atomic clocks have |K| < 1, with the Yb + octupole transition having the highest K = −6 [16]. HCI transitions allow for much higher sensitivities, with |K| > 100 making them particularly attractive candidates for these studies [2][3][4].It was recently shown that coupling of ultralight scalar dark matter to the standard model leads to oscillations of fundamental constants and, therefore, may be observed in clock-comparison experiments [11,17,18]. In addition, dark matter objects with large spatial extent, such as stable topological defects, would lead to transient changes in fundamental constants that are potentially detectable with networks of clocks [12,[19][20][21][22].…”