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Precision spectroscopy of dipole-forbidden rovibrational infrared transitions in molecular ions could serve as a probe for detecting possible temporal variation of fundamental constants and testing fundamental theories [1]. However, until recently, it was impossible to achieve the required precision due to the lack of control over the molecular ions on a single quantum level. We developed new methods which allow us to prepare a single molecular ion in its rovibrational ground state [2], detect its quantum state with high-fidelity [3] and perform highly sensitive and precise spectroscopic experiments on dipole-forbidden infrared transitions in N$_2^+$[4] driven by a quantum cascade laser [5]. The absolute frequency stability of the measurements is provided by referencing all laser frequencies to the Swiss primary frequency standard, the Cs atomic fountain clock FoCS-2, operated by the Swiss Federal Institute of Metrology METAS in Bern [6]. These will allow us to reach an absolute measurement precision level of 10$^{-15}$, establishing new state-of-the-art infrared spectroscopy of the molecular ions and approaching the boundary where BSM physics could be detected.
[1] M. Kajita, G. Gopakumar, M. Abe, M. Hada, and M. Keller. Phys. Rev. A, 89:032509, 2014.
[2] A. Shlykov, M. Roguski and S. Willitsch. In preparation.
[3] M. Sinhal, Z. Meir, K. Najafian, G. Hegi, and S. Willitsch. Science, 367:1213, 2020.
[4] K. Najafian, Z. Meir, and S. Willitsch. Phys. Chem. Chem. Phys., 22:23083, 2020.
[5] M. Bertrand, A. Shlykov, M. Shahmohamadi, M. Beck, S. Willitsch, and J. Faist. Photonics, 9, 2022.
[5] D. Husmann et al. Opt. Express, 29:24592, 2021.
