Highly Charged Ions

Figure 1: Single Ar13+ ion (red cross) implanted in a crystal of fluorescing Be+ ions [1]. It can be seen that the dark highly charged ion strongly repels its singly-ionised neighbours via the Coulomb interaction.

In this project, we aim towards precision laser spectroscopy of dipole-forbidden optical transitions in highly charged ions (HCI) using quantum logic spectroscopy. Due to the high internal fields inside HCI, these transitions show very low sensitivity to the types of external fields that normally limit the accuracy of optical frequency standards. This makes HCI promising candidates for next-generation optical clocks. Furthermore, the large relativistic corrections to the atomic energy levels means that some of these “clock transitions” are exceptionally sensitive to any changes to the fine structure constant α [2], an effect predicted by theories going beyond the Standard Model. Repeated precision spectroscopy over several years will allow new, more sensitive limits to be placed on any variation. .

Dipole-allowed (E1) excitations in HCI lie in the keV energy range, inaccessible for laser spectroscopy. At present, state-of-the-art ultrastable radiation is limited to energies of just a few eV. However, depending on the charge state, fine structure and hyperfine structure magnetic dipole (M1) transitions can be accessed by lasers. Also, higher-order forbidden transitions arise due to the level crossing effect, see Fig. 2.

 

Figure 2: In hydrogen-like ions, levels with the same principal quantum number are degenerate. The energetic order thus follows the Coulomb ordering. Adding more electrons to an ion lifts the degeneracy in angular momentum L due to the shielding effect of the electrons, resulting in the Madelung ordering for neutral atoms. For example, an electron in the 4s orbital is more strongly bound than in the 3d orbital. At some intermediate charge state, the energy difference between the crossing orbitals is within the optical spectrum.

To do precision spectroscopy on HCI they have to be cooled down from their temperature at creation (>1 000 000 K) to their motional ground state in a Paul trap (<0.001 K). We produce highly charged ions in an electron beam ion trap (EBIT) and decelerate them on their way towards a linear Paul trap, where they will be trapped together with laser-cooled beryllium ions. The beryllium ions provide sympathetic cooling, internal state preparation and readout after spectroscopy via the Quantum Logic Spectroscopy scheme. A detailed description of the scheme can be found here.

HCI are very susceptible to charge exchange with residual gas in the vacuum. To reach acceptable storage times, we need a residual pressure below 10-14 mbar. This can only be achieved in a cryogenic environment. We use a closed-cycle pulse tube cryostat to cool our trap region to about 4 K, with a novel coupling technique to minimise the transmission of vibrations to the trap.

This project relies on extensive know-how in many different fields, including cryogenic mechanics and electronics, ion optics, ultrastable lasers, ion trapping and coherent manipulation.

 

References

[1] L. Schmöger et al., Coulomb crystallization of highly charged ions, Science 347.6227 (2015): 1233-1236.

[2] J.C. Berengut et al., Electron-hole transitions in multiply charged ions for precision laser spectroscopy and searching for variations in α, Phys. Rev. Lett. 106.21 (2011): 210802.

 

The Team

from left to right: Piet O. Schmidt, Tobias Leopold, Peter Micke, Steven King

In collaboration with the Max-Planck-Institut für Kernphysik, Heidelberg:

José Ramon Crespo López-Urrutia