Precision spectroscopy on single cold trapped molecular nitrogen ions

The ability to precisely control and manipulate single cold trapped particles has enabled spectroscopic studies on narrow transitions of ions at unprecedented levels of precision\footnote{P. O. Schmidt \textit{et. al.}, \textit{Science} \textbf{309} (2005), 749.}. This has opened up a wide range of applications, from tests of fundamental physical concepts, e.g., possible time-variations of fundamental constants, to new and improved frequency standards. So far most of these experiments have concentrated on atomic ions. Recently, however, attention has also been focused on molecular species, and molecular nitrogen ions have been identified as promising candidates for testing a possible time-variation of the proton/electron mass ratio\footnote{M. Kajita \textit{et. al.}, \textit{Phys. Rev. A} \textbf{89} (2014), 032509.}. Here, we report progress towards precision-spectroscopic studies on dipole-forbidden vibrational transitions in single trapped N$_{2}^{+}$ ions\footnote{M. Germann , X. Tong, S. Willitsch, \textit{Nature Physics} \textbf{10} (2014), 820.}. Our approach relies on the state-selective generation of single N$_{2}^{+}$ ions\footnote{X. Tong, A. Winney, S. Willitsch, \textit{Phys. Rev. Lett.} \textbf{105} (2010), 143001.}, subsequent infrared excitation using high intensity, narrow-band quantum-cascade lasers and a quantum-logic scheme for non-destructive state readout. We also characterize processes limiting the state lifetimes in our experiment, which impair the measurement fidelity