ABSTRACT Title of dissertation: A RING WITH A SPIN: SUPERFLUIDITY IN A TOROIDAL BOSE-EINSTEIN CONDENSATE Anand Krishnan Ramanathan, Doctor of Philosophy, 2011 Dissertation directed by: Steve Rolston Chemical Physics Program Superfluidity is a remarkable phenomenon. Superfluidity was initially charac- terized by flow without friction, first seen in liquid helium in 1938, and has been studied extensively since. Superfluidity is believed to be related to, but not iden- tical to Bose-Einstein condensation, a statistical mechanical phenomena predicted by Albert Einstein in 1924 based on the statistics of Satyendra Nath Bose, where bosonic atoms make a phase transition to form a Bose-Einstein condensate (BEC), a gas which has macroscopic occupation of a single quantum state. Developments in laser cooling of neutral atoms and the subsequent realization of Bose-Einstein condensates in ultracold gases have opened a new window into the study of superfluidity and its relation to Bose-Einstein condensation. In our atomic sodium BEC experiment, we studied superfluidity and dissipationless flow in an all-optical toroidal trap, constructed using the combination of a horizontal “sheet”-like beam and vertical “ring”-like beam, which, like a circuit loop, allows flow around the ring. On inducing a single quantum of circulation in the condensate, the smoothness and uniformity of the toroidal BEC enabled the sustaining of a persistent current lasting 40 seconds, limited by the lifetime of the BEC due to background gas pressure. This success set the stage for further experiments studying superfluidity. In a first set of experiments, we studied the stability of the persistent current by inserting a barrier in the flow path of the ring. The superflow stopped abruptly at a barrier strength such that the local flow velocity at the barrier exceeded a critical velocity, which supported decay via the creation of a vortex-antivortex pair. Our precise control in inducing and arresting superflow in the BEC is a first step toward studying other aspects of superfluidity, such as the effect of temperature and dimensionality. This thesis discusses these experiments and also details partial-transfer ab- sorption imaging, an imaging technique developed in the course of this work. A Ring with a Spin : Superfluidity in a toroidal Bose-Einstein condensate by Anand Krishnan Ramanathan Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2011 Advisory Committee: Professor Steven Rolston, Chair Doctor Gretchen Campbell Professor Kristian Helmerson Professor Wendell Hill Professor Christopher Lobb, Deans’s Representative Professor William Phillips c Copyright by Anand Ramanathan 2011 To my Father, who has been a source of inspiration and strength throughout my education and my life so far. Acknowledgments I owe my gratitude to all the people who have made this thesis possible and because of whom my graduate experience has been one that I will cherish forever. The Laser Cooling and Trapping (LC) Group at NIST has been a fantastic place to work at, and I am indebted for the extremely friendly and helpful atmosphere, and all the people whom I have had a chance to interact with. First, I would like to thank my advisors Kris Helmerson and Gretchen Camp- bell. I feel very fortunate to have had the opportunity to interact and to learn from both of them. The support and encouragement shown by Kris was crucial during my first few months at NIST. Kris has always shown the readiness to help right from choosing the right conference to attend to setting things right in the lab. I have admired Kris for his diverse interests, from BEC physics to biophysics, and for the many collaborators he has around the world. Gretchen, who joined the group in my fourth year, has been immensely helpful during the later part of my PhD, and more than once, has prevented me from making a mistake that would have been detrimental to my career. She has always gone out of the way to make sure that I got the help and advice that I needed. She has been key in checking on the progress of my thesis and setting goals for completion. She has given prompt feedback on anything I have sent her. I’ve always admired how she has been on top of things, and follows up on even the smallest thing such as a question about Latex, despite her several commitments at NIST and UMD and her new Strontium lab. iii Bill Phillips has been the biggest inspiration through my PhD. His enthusiasm for physics has always been infectious, and kept me going when nothing seemed to work int the lab. He has asked some of the best questions, which have stimulated many thought-provoking discussions. Bill started our group’s Wednesday meetings, where we met with Chris Lobb and Wendell Hill and others from UMD. Although I’ve often felt that the discussions in such meetings led to nowhere, they fostered our best collaboration, without which a lot of this work would not have happened. Bill has the amazing ability to reach out to people from different backgrounds, and stimulate a discussion that brings about interesting physics. As a group leader, Bill has also been a role model in many ways. He is always approachable even when his schedule is busy. He takes time to give some carefully thought out advice. He is encouraging of everyone. I feel very fortunate to have discussed physics with Chris Lobb. I started reading about superconductivity during my third year under his guidance, and en- joyed our discussions on the subject. He always found time to guide me through Tinkham’s book, despite his hectic schedule, and was very patient through my slow learning of the subject. The little I learned from that time has gone a long way in giving me a different perspective on the physics we have seen. I have also enjoyed the many discussions I have had with Chris at his office. He has been remarkable in the number of wonderful insights he has come up with. Despite that, he has always been keenly aware of any small contribution by someone else and has been very generous in giving credit for it. I have also benefited from Wendell Hill and Steve Rolston. Wendell has always iv kept in touch with what I am doing, and has given me useful advice along the way. Steve has had an open door for discussing physics or my thesis and has provided me invaluable administrative support to ensure that I have had no problems in working at NIST, which is off-campus from UMD. During my time at NIST, I have had the fortune of working with several ex- tremely talented post-docs, Mikkel Andersen, Changhyun Ryu, Pierre Clad´e, S´ergio Muniz and Kevin Wright, who have helped me along the way. Mikkel got me started into the lab, helping me with my initial experiment. Chang helped me greatly in understanding the complicated BEC apparatus. He also shared with me, what he had learned about the field, to try and give me a broader perspective. Pierre got me started on analyzing data, which has stood me good right through my PhD. I cannot forget our discussions on BKT physics, which got me enthusiastic about the physics, and along with that, integrated into the science of the experiment. I am grateful to Pierre and Chang into taking care to see that I had a defined PhD project before they left. I have worked the longest with S´ergio, and we have been through many battles together. S´ergio is one of the warmest and most helpful people I’ve known. He has taught me a lot of electronics, and an amazing array of other miscellaneous lab- related things. A lot of my skills as an experimentalist were built with him looking over me. I probably would not be writing this thesis at this point if not for Kevin. Kevin gave the lab its critical mass, and along with S´ergio, we had an extremely productive year, which brought out the best in all of us. From Kevin, just when I v thought I knew everything that was there in the lab, I ever so often learned how something could be done better. I have admired his meticulousness and tidiness in working. His knowledge of optics, and his ability to explain it is phenomenal. I have also learned from him, how to be generous with credit. Apart from post-docs, our lab had several visitors who helped along the way. Of them, I learned a lot from fellow graduate students Russ Anderson and Martin Zelan, who showed me that a lot could be done even in a short visit. I also had the fortune of working with Vasant Natarajan, whom I had known since my undergrad- uate days. He has provided excellent mentorship right through, and has played an important role in shaping my career. There are many other people in the LC Group who have helped me along the way. Most notable is the other sodium BEC lab, with Paul Lett, Lincoln Turner, Yingmei Liu, Jianing Han and Jonathan Wrubel. Rani Kishore of the Tweezer lab has also helped me in many ways. I greatly cherish sharing my office with Brooke Hester (previously Cranswick), who provided great company through the years. She was quick to tell me about any opportunity that could be of use to me, and got me involved with the OSA and other activities at UMD, which broadened my horizon beyond the confines of my research. She always there when I had a bad day or when I was worried about something.