Direct Observation of the $6S_{1/2}$ to $5D_{3/2}$ Electric Quadrupole Transition in Barium-138*

The $6S_{1/2}$ to $5D_{3/2}$ electric quadrupole transition at 2051 nm in Ba+ plays an important role in a number of proposed experiments.\footnote{K. Beloy, et. al. arXiv:0804.4317v1 [physics.atom-ph] 2008}$^,$\footnote{J. Sherman, et. al. arXiv:physics/0504013v2 [physics.atom-ph] 2005}$^,$\footnote{E. N. Fortson. Phys. Rev. Lett., 70(16):2383-2386, Apr 1993} We present the results of the first narrow laser spectroscopy performed on this transition. 2051 nm light is generated by a diode pumped solid state Tm,Ho:YLF laser. The laser is frequency stabilized to a high finesse cavity made from ultra-low expansion glass. In order to take advantage of higher performing optics and detectors available at shorter wavelengths, the 2051 nm light is frequency doubled using a periodically poled lithium niobate crystal inside a bow-tie enhancement cavity before being sent to the reference cavity. Using this laser system we observed Rabi oscillations on the $6S_{1/2}$ to $5D_{3/2}$ transition and demonstrated a laser-ion coherence time of 3 ms.