“Suitability of Linear Quadrupole Ion Traps for Large Coulomb Crystals,” D.A. Tabor, V. Rajagopal, Y-W. Lin and B. Odom. Appl. Phys. B. 107, 1097-1104 (2012) [pdf]
Growing and studying large Coulomb crystals, composed of tens to hundreds of thousands of ions, in linear quadrupole ion traps presents new challenges for trap implementation. We consider several trap designs, first comparing the total driven micromotion amplitude as a function of location within the trapping volume; total micromotion is an important point of comparison since it can limit crystal size by transfer of radiofrequency drive energy into thermal energy. We also compare the axial component of micromotion, which leads to first-order Doppler shifts along the preferred spectroscopy axis in precision measurements on large Coulomb crystals. Finally, we compare trapping potential anharmonicity, which can induce nonlinear resonance heating by shifting normal mode frequencies onto resonance as a crystal grows. We apply a non-deforming crystal approximation for simple calculation of these anharmonicity-induced shifts, allowing a straightforward estimation of when crystal growth can lead to excitation of different nonlinear heating resonances. In the axial micromotion and anharmonicity points of comparison, we find significant differences between the compared trap designs, with an original rotated-endcap trap performing slightly better than the conventional in-line endcap trap.
“Broadband Optical Cooling of Molecular Rotors from Room Temperature to the Ground State,” C.-Y. Lien, C.S. Seck, Y.-W. Lin, J.H.V. Nguyen, D.A. Tabor, and. B.C. Odom. Nature Comm. 5, 4783 (2014) [pdf]
Laser cycling of resonances can remove entropy from a system via spontaneously emitted photons, with electronic resonances providing the fastest available cooling timescales because of their rapid relaxation rates. Although atoms are routinely laser cooled using electronic resonances, cooling of even simple diatomic molecules faces two challenges: every populated rotational and vibrational state requires a different laser frequency, and electronic relaxation generally excites vibrations. Here, we cool trapped AlH+ molecules to their ground rotational-vibrational quantum state using a single spectrally filtered broadband laser electronically exciting many rotational states. Undesired vibrational excitation is avoided because of vibrational-electronic decoupling in AlH+. We demonstrate rotational cooling on the 140(20) ms timescale from room temperature to 3.8 (+1.3/-0.3) K, corresponding to a ground state population increase from ∼3% to 95 (+1.3/-2.0) %. This cooling technique could be applied to several other known ionic and neutral molecular species of interest for quantum information processing, ultracold chemistry applications, and precision tests of fundamental symmetries.
“Toward Rotational Cooling of Trapped SiO+ by Optical Pumping,” D.A. Tabor, Doctoral Thesis (2014) [pdf]
This thesis presents a scheme for preparation of trapped molecular ions with a high degree of internal state purity by optical pumping with a broadband pulse-shaped femtosecond laser; the internal structure of SiO+ permits fast stepwise pumping through the tens of rotational levels populated in a room-temperature distribution. Two analyses, which guided the experimental implementation, are presented: (1) a novel method of quantify- ing anharmonicity in the trapping potentials, which limits the number of ions that can be trapped, and (2) a rate-equation simulation of the quantum state evolution during pumping. Experimental implementation of pulse shaping and its characterization are discussed, as is the molecular spectroscopy used to reference this light to the rotational cooling transitions. Internal state analysis can be performed using resonance enhanced multiphoton dissociation.