Removing Trapped Rubidium through Optical Pumping at Large Magnetic Fields by Conrad Newfield B.S., Engineering Physics, University of Colorado Boulder B.S., Applied Mathematics, University of Colorado Boulder Defense Date: 04/13/2020 Research Advisor: Jun Ye (Physics) Honors Council Representative: Paul Beale (Physics) Committee Member: Michael Litos (Physics) Committee Member: Brian Zaharatos (Applied Mathematics) A thesis submitted to the Faculty of the University of Colorado in partial fulfillment of the requirements for the award of Departmental Honors in the Department of Physics 2020 This thesis entitled: Removing Trapped Rubidium through Optical Pumping at Large Magnetic Fields written by Conrad Newfield has been approved for the Department of Physics Prof. Jun Ye Prof. Michael Litos Prof. Paul Beale Prof. Brian Zaharatos Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Newfield, Conrad (B.S., Engineering Physics) Removing Trapped Rubidium through Optical Pumping at Large Magnetic Fields Thesis directed by Prof. Jun Ye Ultracold dipolar molecules like 40K87Rb are ideal candidates to study phenomena in quantum chemistry and many-body physics. The additional rotational and vibrational degrees of freedom of molecules make them interesting species to study but also create huge technological challenges. Due to the experimental complexity and large number of cooling steps required for molecule production at nanokelvin temperatures, the final molecule count for our experiment fluctuates and composes only a small fraction of the initial atom count. Blasting away excess atoms during initial molecule production is key to augmenting number consistency. KRb will be destroyed if unconverted K and Rb atoms are not removed quickly enough. To improve molecule number and stability, a method of blasting Rb at high magnetic fields was tested by optically pumping rather than using radio- frequency adiabatic rapid passages (ARPs). Ground-state KRb lifetime was found to be the same for both methods. This thesis describes this attempt at optical pumping, its theoretical basis, and results. Dedication To friends, at CU and afar, family, and clean basalt geometry. v Acknowledgements My journey to end to up working in a physics lab started early in life. Several incredible teachers guided me and encouraged my interest in math and physics from a young age. Specifically I would like to thank Laura Cummings, auntie Betsy Hatter, and Tiffany Coke. I certainly wouldn't have ended up in Boulder without my passion for climbing, so I must thank the Hawai`i climbing community. I would like to thank Michael Litos for guiding me throughout college and helping me get a position in one of Jun Ye's labs. I would of course like to thank Jun because none of this work would have been possible without his belief in me. Jun's work ethic is otherworldly and I feel incredibly lucky to have been able to work under him. I can't wait to see all the groundbreaking research that comes out of his labs in the next decades. I would also like to thank the graduate students and postdocs who served as my mentors in lab: Kyle Matsuda, Will Tobias, Jun-Ru Li, Giacomo Valtolina and Luigi De Marco. Kyle's optics knowledge and happiness made lab a joy to work in. Will's electronics knowledge and guidance with projects saved me many times. Jun-Ru's ability to explain what was going in lab at all times was incredible. Giacomo's deep familiarity with the experiment and work ethic inspires me. I especially would like to thank Luigi who guided me so much in lab, helping me whenever I got stuck, explaining any physics concept in an understandable way, and even coming in to lab on multiple weekends to help me finish this project. His efforts truly made this project successful and he's the best lab pong doubles partner anyone could ask for. I would like to thank my friends all over the country, especially those who helped me survive vi high school and college. I wouldn't be where I am today without all of you. Thank you especially to Albert H. W. Jiang and Caitlin Steele for always being there for me even in my darkest times. Lastly, I would like to thank my family for their constant support me. Their sacrifices changed the course of my life for the better from an early age. vii Contents Chapter 1 Introduction 1 2 Theory, apparatus and motivation 3 2.1 Atomic structure . .3 2.2 Bosons and fermions . .4 2.3 Blasting atoms . .4 2.3.1 Rubidium D2 line . .4 2.4 Optical pumping theory . .6 2.4.1 Hyperfine splitting . .6 2.4.2 Transition strengths . .9 2.4.3 Transition linewidths . 10 2.5 Laser systems & cooling process . 10 2.5.1 Magneto-optical trap and molasses . 13 2.5.2 Plugged quadrupole and magnetic evaporation . 16 2.6 Optical evaporation . 17 2.7 Molecule production . 17 2.7.1 Magneto-association . 17 2.7.2 STIRAP . 18 2.7.3 Imaging molecules . 18 viii 2.7.4 Quantum degeneracy . 18 2.8 Molecule reactions . 19 2.8.1 KRb + KRb . 19 2.8.2 Molecule-atom reactions . 20 3 Laser system 22 3.1 Laser locking . 22 3.1.1 Laser details . 22 3.1.2 Temperature control . 23 3.1.3 Locking method . 24 3.1.4 Physical setup . 24 3.1.5 Electronics: locking . 28 4 Results 29 4.1 Lineshape of KRb . 29 4.2 Lifetime measurement . 31 4.3 Molecule number as a function of pulse time . 32 4.4 Lineshape on Rb . 33 5 Conclusion 35 5.1 Future directions . 35 Bibliography 36 Appendix A Jupyter notebook for transition frequencies, strengths, and linewidths 38 ix Figures Figure 2.1 Rb D2 line data . .5 2.2 Ground-state Zeeman shifts . .8 2.3 Excited-state Zeeman shifts . .8 2.4 Relative transition strengths . .9 2.5 Relative transition strengths against frequency . 11 2.6 Transition linewidths . 12 2.7 Experiment steps . 14 2.8 MOT setup . 15 2.9 KRb donut . 21 3.1 Laser linewidth . 23 3.2 TEC replacement . 25 3.3 Experimental setup of high-field Rb laser . 26 3.4 Light to science cell . 27 4.1 Molecule lineshape . 30 4.2 Molecule lifetime . 31 4.3 Pumping over time . 32 4.4 Rubidium lineshape . 33 4.5 Rubidium lineshape (low power) . 34 Chapter 1 Introduction In recent years, researchers have been increasingly interested in ultracold gases of molecules. Progress has been achieved in creating ever denser and colder gases with the end goal of achiev- ing quantum degeneracy [1]. The motivation for this work is clear: ultracold, degenerate polar molecules can provide new insights into the behavior of particles in the quantum regime. They are ideal systems for tests of many branches of quantum mechanics including quantum information science [2], ultracold chemistry [1], quantum simulation [3], and low-dimension dynamics [1]. Polar molecules have stronger long-range dipolar interactions and more degrees of freedom compared to atoms. The added complexity of molecules over atoms means existing cooling tech- niques must be adapted and new ones created. Direct laser cooling of molecules has been demon- strated for certain, specially-chosen molecules but the phase-space density is nowhere near that needed for degeneracy [4]. An alternate method, used in our lab, continues work done on ultracold atoms by associating them into weakly-bound Feshbach molecules and then coherently transferring them to the molecular ground state. Recently, in a huge milestone and technological achievement, the first degenerate gas of polar molecules was created at JILA using ultracold potassium-rubidium [5]. Production of increasingly degenerate gases is limited by inefficiencies throughout the cooling and molecule creation process. For example, the conversion efficiency from atoms to molecules is less than 50%, leaving a significant number of K and Rb atoms in the trap that destroy molecules quickly, limiting the time for experiments. The two atomic species will react with KRb, causing 2 heating and molecule loss. To blast away excess K, a resonant light pulse is used on an optical cycling transition. However, Rb atoms are not in a state with such a cycling transition, and so it is necessary to change the atomic hyperfine state to remove them. To accomplish this, we have typically used a series of microwave adiabatic rapid passages (ARPs), each followed by a pulse of resonant light, to remove a majority of trapped Rb. The time duration and non-unity efficiency of the ARPs + blast sequence was theorized by our lab as a contributor to molecule number fluctuation between experimental runs. To improve molecule number and stability, a laser system was constructed to optically pump Rb into a state from which resonant light is used to complete Rb removal. A frequency shift from the zero-field transition on the D2 was calculated to account for the high magnetic fields at which atomic removal occur. This thesis details our first attempt at optically pumping Rb at high magnetic fields. In Chapter 2, I give a motivation for my work along with a brief overview of atomic physics, followed by a discussion on the theoretical basis for high-field optical pumping of Rb. I then introduce the experimental apparatus used to create 40K87Rb, highlighting the cooling and molecule production steps to which my laser applies. In Chapter 3, I explain the setup of my laser system, detail the locking mechanism and discuss issues encountered along the path of construction. In Chapter 4, I present the results of my work on laser lineshape and molecule lifetime, discussing differences with the current method of numerous ARPs.