Speaker
Description
High resolution spectroscopy of molecules is a prime candidate to measure potential temporal changes in the proton-to-electron mass ratio, μ [1]. These potential changes can be detected by comparing vibrational or rotational transitions in molecules to optical atomic transitions.
In our experiment, a vibrational Raman transition in a nitrogen ion will be compared to a quadrupole transition in a calcium ion. The N$_2$$^+$ ion has systematic shifts better than the best optical atomic clocks to date. To perform precision spectroscopy, a single nitrogen ion will be co-trapped in a linear Paul trap with a $^{40}$Ca$^+$ ion. This calcium ion will act as a frequency reference and be used for the cooling and state detection of the nitrogen ion.
Prerequisite to this is the preparation of $^{14}$N$_2$$^+$ in a specific rovibronic state. Recently, a 2+1’ resonance-enhanced multiphoton ionisation (REMPI) scheme was developed, using the a$^1$Σ$_g$$^+$(ν=6) ← X$^1$Σ$_g$$^+$(ν=0) band in $^{14}$N$_2$ for the resonant excitation. This scheme demonstrated a fidelity of >99% for loading into the rovibronic ground state [2]. However, simulations show that the high amplitude and inhomogeneous electric fields of the ion trap broaden the ionisation threshold and prevent state-selective loading in many cases. Rapidly switching the trap off during loading can reduce the electric field and can mitigate this to allow state selective loading of the ion trap [3].
[1] M. Kajita et al., Physical Review A 89, 032509 (2014).
[2] A. Gardner et al., Scientific Reports 9, 506 (2019).
[3] L. Blackburn et al., Scientific Reports 10, 18449 (2020).
