Internal and External State Preparation of a Single Molecule

Dragan, James

doi:https://doi.org/10.21985/n2-8j36-9s13

Work

Internal and External State Preparation of a Single Molecule

Public

Download PDF

Download All Files (.zip)

The vibrational and rotational degrees of freedom in a molecule add complexity that complicate optical cycling and state preparation procedures. However, the same additional degrees of freedom also make molecules suitable candidates for tests of physics beyond the standard model. A precision measurement of the transition energy between two vibrational levels can set a limit on the time variation of the proton-to-electron mass ratio. If that experiment is made in an ion trap, spectroscopy using quantum logic techniques can be employed. These methods allow for the internal state of the molecule to be mapped onto a shared motional mode between the molecule and an auxiliary atomic ion. This is only possible when the motional modes are first cooled to the ground state. Typically the auxiliary atom is used for sympathetic cooling and state readout, as the molecule lacks a closed optical transition to laser cool on. Preparation of the motional modes can become challenging when the two ions have different masses, and only one ion is actively Doppler cooled. Reduction of the second ion's thermal motion by sympathetic cooling is less efficient and the two ion crystal is more susceptible to heating sources that can potentially thermalize the ions and interrupt data collection. This thesis outlines the experimental control needed to fully prepare a two ion crystal with a mass ratio of 1:3, when only one ion is Doppler cooled. A scheme in which the ion crystal is fully and automatically prepared in several seconds after melting, including the appropriate trap settings and additional lasers is discussed. Ground rovibrational state preparation of the \SiO molecule is achieved by optical pumping on the X$^2\Sigma^+$-B$^2\Sigma^+$ transition. It is estimated that 68(6)\% of the population is pumped into N=0, with the remaining population in N=1, where N is the spinless rotational quantum number. Obtaining ground rovibrational state preparation is the first step towards survey and precision spectroscopy of vibrational overtone transitions in the X$^2\Sigma^+$ state. The optical pumping process of \SiO by driving the X-B transition is modeled using rate equations and the intervening A$^2\Pi$ state is found to be vital by providing a pathway for state parity to change. The simulation results suggest that the B-A transition strength is larger than the highest level calculations predict. In addition, calculations of the transition dipole moments (TDMs) show that the excited B state is more likely to decay on vibrational cooling transitions than on heating ones. To gain insight, a simple model is introduced and the results show that a combination of terms lead to a stronger TDM for vibrational cooling decays. The same terms fortuitously cancel and lead to a lower TDM for vibrational heating decays. The result is that optical cycling on the X-B transition naturally leads to vibrational cooling. In a first step towards nondestructive state readout of a molecule, a two ion crystal of \Ba and \SiO are cooled to the ground motional state in three dimensions using EIT sideband cooling. Along the axial direction, the resulting ground state occupations are $\bar{n}_{oi,z} = 0.06(3)$ and $\bar{n}_{op,z} = 0.08(3)$. With ground state cooling operational, the attempts at the first demonstration of Photon Recoil Readout (PRR) are shown. This state dependent heating technique works by synchronizing photon recoils from stimulated absorption in the molecule, with a specific motional mode. If this transition is driven, momentum is coherently added to a given motional mode, which pumps population out of the ground motional state. The theory behind PRR, initial attempts and potential improvements are discussed.

Creator