Toward XUV frequency comb spectroscopy of the 1 S–2 S transition in $$\hbox {He}^+$$
Jorge Moreno, Fabian Schmid, Johannes Weitenberg, Savely G. Karshenboim, Theodor W. Hänsch, Thomas Udem, Akira Ozawa
Abstract
Abstract The energy levels of hydrogen-like atoms and ions are accurately described by bound-state quantum electrodynamics (QED). $$\hbox {He}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>He</mml:mtext> <mml:mo>+</mml:mo> </mml:msup> </mml:math> 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 $$\hbox {He}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>He</mml:mtext> <mml:mo>+</mml:mo> </mml:msup> </mml:math> ions. Measuring a transition in $$\hbox {He}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>He</mml:mtext> <mml:mo>+</mml:mo> </mml:msup> </mml:math> 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 $$\hbox {He}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>He</mml:mtext> <mml:mo>+</mml:mo> </mml:msup> </mml:math> . 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 $$\hbox {He}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>He</mml:mtext> <mml:mo>+</mml:mo> </mml:msup> </mml:math> ions are trapped in a Paul trap and sympathetically cooled with laser-cooled $$\hbox {Be}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>Be</mml:mtext> <mml:mo>+</mml:mo> </mml:msup> </mml:math> ions. Our recently developed signal detection scheme based on secular-scan spectrometry is capable of detecting $$\hbox {He}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>He</mml:mtext> <mml:mo>+</mml:mo> </mml:msup> </mml:math> excitation with single-event sensitivity. Graphic abstract