Quantum Control of a single H2+ molecular ion

Colloquium by Dr. Daniel Kienzler (ETH Zürich, Schweiz) at PTB Braunschweig, Kohlrausch- Bau, Hörsaal Room 135, 38116 Braunschweig

Abstract:

I will present our latest results, implementing pure quantum state preparation, coherent manipulation, and non-destructive state readout of the hydrogen molecular ion H2+ [1]. The hydrogen molecular ion is the simplest stable molecule, and its structure can be calculated ab initio to high precision. However, challenging properties such as high reactivity, low mass, and the absence of rovibrational dipole transitions have thus far strongly limited spectroscopic studies of H2+.

We trap a single H2+ molecule together with a single beryllium ion using a cryogenic Paul trap apparatus, achieving trapping lifetimes of 11 h and ground-state cooling of the shared axial motion [2]. With this platform, we have recently implemented Quantum Logic Spectroscopy of H2+. We utilize helium buffer-gas cooling or 2+1 REMPI to prepare selected rotational states of H2+ in its vibrational ground state. We combine this with quantum-logic operations between the molecule and the beryllium ion for the preparation of single hyperfine states and non-destructive readout, achieving a combined state-preparation and readout fidelity of 66.5(8)%. We demonstrate Rabi flopping on several hyperfine transitions using stimulated Raman transitions and microwaves.  I will further present recent progress on high-precision hyperfine spectroscopy of ortho-H2+ (rotation L=1, vibration ν=0), where we achieve a precision of 50 mHz with a 6-minute averaging time and estimate a preliminary accuracy on the order of 100 mHz. This could provide a stringent test of state-of-the-art molecular theory and put an improved bound on a possible new tensor force between the two constituent protons of H2+ [3].

Our results pave the way for many high-precision spectroscopy studies of H2+, which would enable tests of QED, metrology of fundamental constants, and the implementation of an optical molecular clock based on the simplest molecule in nature.  

1. D. Holzapfel et al., Phys. Rev. X 15, 031009 (2025).
2. N. Schwegler et al., Phys. Rev. Lett. 131, 133003 (2023).
3. N. F. Ramsey, Physica 96A, 285 (1979).

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