No abstract

While it is straightforward to count laser-cooled trapped ions by fluorescence imaging, detecting the number of dark ions embedded and sympathetically cooled in a mixed ion crystal is more challenging. We demonstrate a method to track the number of dark ions in real time with singleparticle sensitivity. This is achieved by observing discrete steps in the amount of fluorescence emitted from the coolant ions while exciting secular motional resonances of dark ions. By counting the number of fluorescence steps, we can identify the number of dark ions without calibration and without relying on any physical model of the motional excitation. We demonstrate the scheme by detecting H + 2 and H + 3 ions embedded in a Be + ion Coulomb crystal in a linear radio frequency trap. Our method allows observing the generation and destruction of individual ions simultaneously for different types of ions. Besides high-resolution spectroscopy of dark ions, another application is the detection of chemical reactions in real time with single-particle sensitivity. This is demonstrated in this work.

The energy levels of hydrogen-like atoms and ions are accurately described by bound-state quantum electrodynamics (QED). $$\hbox {He}^{+}$$ He + ions have a doubly charged nucleus, which enhances the higher-order QED contributions and makes them interesting for precise tests of QED. Systematic effects that currently dominate the uncertainty in hydrogen spectroscopy, such as the second-order Doppler shift and time-of-flight broadening, are largely suppressed by performing spectroscopy on trapped and cooled $$\hbox {He}^{+}$$ He + ions. Measuring a transition in $$\hbox {He}^{+}$$ He + will extend the test of QED beyond the long-studied hydrogen. In this article, we describe our progress toward precision spectroscopy of the 1 S–2 S two-photon transition in $$\hbox {He}^{+}$$ He + . The transition can be excited by radiation at a wavelength of 60.8 nm generated by a high-power infrared frequency comb using high-order harmonic generation (HHG). The $$\hbox {He}^{+}$$ He + ions are trapped in a Paul trap and sympathetically cooled with laser-cooled $$\hbox {Be}^{+}$$ Be + ions. Our recently developed signal detection scheme based on secular-scan spectrometry is capable of detecting $$\hbox {He}^{+}$$ He + excitation with single-event sensitivity. Graphic abstract