We describe a simple method for measuring the residual fast phase noise of a cavity-stabilized laser using the cavity as a reference. The method is based on generating a beat note between the laser output and the strongly filtered light transmitted through the cavity. The beat note can be directly analyzed without requiring further calibration of system parameters. We apply the method to measure the residual phase noise of an external-cavity diode laser (ECDL) locked to a reference cavity and compare the results with an analysis of the in-loop error signal of the feedback system.

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.

Laser-induced energy deposition has been proposed for an effective flow control concept in super-and hypersonic transportation. Especially for blunt bodies the strength of the normal shock waves can be significantly mitigated by a modification of the gas temperature commonly generated by spark-discharge or repetitive laser-induced gas breakdown. The paper presents experimental and computational results of a laser-induced, non-repetitive gas heating concept in supersonic argon flow. Ignited by the gas breakdown of a Q-switched Nd:YAG laser a focused continuous wave CO2 laser sustains a plasma. The pressure distribution of a miniature hemisphere is measured by four static pressure taps and determines a drag reduction of up to 55 % for the Mach number 2.1 and of up to 60 % for the Mach number 2.7 using an average pulse power of up to 5.4 kW for a typical duration of 700 µs.

We investigate the motional resonances of dual-species Coulomb crystals comprised of 9 Be + and 24 Mg + ions held in a 4-rod linear Paul trap. Our experimental data and simulations show that the secular motion of such mixed crystals has rich dynamics. Their secular spectra can differ significantly from those of pure ion crystals. We propose a simple model based on mechanical coupling with Coulomb interactions between the two different ion species that explains many features of the secular spectrum. Our findings contribute to a more reliable identification of the ion species in mixed crystals.

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