In this project, carefully controlled lasers will be used to probe the detailed atomic structure of a class of heavy atoms including thallium and indium with unprecedented precision. Atoms, quintessential quantum mechanical objects, have long been used to confirm and test quantum theory. But in recent years, with advances in laser, optical, and signal processing technology, extremely precise experiments on certain heavy atoms have yielded insights into physics more commonly associated with elementary particles and large accelerators. Such tests of fundamental particle physics in these "table-top" experiments can only occur through both precise experiments and also sophisticated quantum mechanical calculations of these many-electron systems. Experiments in this laboratory using atoms in both heated vapor cells and atomic beam apparatus will be completed to test the accuracy of this cutting-edge atomic theory. Finally, an experiment will probe possible violations of "time-reversal" invariance in thallium atoms, which would be the signal for new physics beyond the current Standard Model of particle physics.
At Williams College, these experiments serve as ideal research training for undergraduate physics students at every stage of their education. In the absence of graduate students, these students grow to become fully engaged junior colleagues. Since NSF support for this work began in 1998, a dozen students have become co-authors on journal publications, and twenty young scientists who received their first exposure to research in this laboratory have completed or are now enrolled in Ph.D. programs in physics and related fields. Students take active, central roles in completing laboratory projects through which they develop expertise with optics and lasers, electronics and control systems, as well as sophisticated data analysis procedures. Because of the relatively small physical scale of the research, they can appreciate the entirety of the experimental effort, gain valuable, specific experimental skills, and yet participate in an exciting cutting-edge research field which uses atoms to test physical theory at its most basic level. A continuing effort will be made to include students in this research at this crucial early point in their undergraduate years, as they begin to make longer-term career choices.
This research program, located at an undergraduate institution, involves table-top experiments with lasers and atoms ultimately designed to provide insights into the so-called "Standard Model of particle physics" that is typically studied by physicists at large particle accelerator facilities. Such low-energy physics tests complement accelerator-based experimental work. The small size of these effects demands very high precision, extensive study of systematic errors, and careful experimental design. In the present work, properties of the metal atoms thallium and indium are measured to very high accuracy. Results have been compared to state-of-the-art theoretical calculations of these same properties, as high-quality tests of the fundamental physics phenomena probed by these measurements rely critically on the combination of precise experimental results reinforced by accurate, independent atomic structure calculations. Williams undergraduate students become involved in all aspects of the experimental work, designing and testing laser, optical, and signal processing systems, and carrying out data collection and data analysis procedures. In the course of this high-precision work, the group developed a number of novel signal-processing and laser stabilization techniques which are broadly-applicable with the laser and atomic physics community, and have been written up as manuscripts for journals focused on experimental physics 'methods'. During the course this NSF-funded grant cycle, The PI and his students completed two diode laser spectroscopy measurements of the atomic properties of Group IIIA atoms (thallium, indium) which can be compared to state-of-the-art atomic theory calculations. These atoms contain three valence electrons, challenging the approximation techniques required to accurately compute the quantum mechanical behavior within these atoms. This experiment-theory interplay has resulted in significantly improved accuracy for both in recent years, The atoms were used both in heated vapor cells as well as a dense, collimated atomic beam apparatus. In the first experiment, two lasers, one in the near-ultraviolet (UV), and one in the near-infrared (IR) wavelength range are directed into a heated quartz vapor cell of thallium. The UV laser is frequency-stabilized and causes the valence electron in thallium to be excited to a higher-lying energy level. The IR laser then promotes this electron to higher-excited state, whose detailed 'hyperfine' structure was studied at unprecedented levels of precision. Furthermore, the researchers confirmed a substantial error in an older measurement of this quantity, and have demonstrated much improved agreement with the theoretical prediction for this hyperfine structure. The basic technique using two lasers to excite these atoms in two steps will continue to be used to study other excited states of thallium and indium. Three senior honors students at Williams completed theses focused on the development, implementation, and completion of this experiment. Two are co-authors on the 2014 Physical Review article describing this work. The second experiment made use of our evacuated atomic beam apparatus, in which we heated a sample of indium metal atoms to 900 deg. C, and directed them in a collimated 'beam' towards an interaction point where a blue diode laser beam intersected the atoms. At that location, we apply very large electric fields to the atomic sample (5 - 10 kilovolts/cm). The large perturbation shifts the energy levels of the indium atoms in a way that can be theoretically predicted, given a model of the atom's valence electron wave function. We measured the small shift in energy (about 1 part per million of the total energy difference) by using the laser to excite the atoms and precisely determining the shift in the peak of the atomic resonance. We were able to measure the so-called "Stark" shift to 3 parts in a thousand -- 30 times more precisely than previous experimental work. Independently, our theory collaborators computed this shift and found a result in excellent agreement with our experimental value. Three Williams students worked on this spectroscopy experiment as their senior honors thesis project, and several other younger students contributed during the course of this experiment. The students were involved in vacuum and source oven design, laser stabilization and calibration, development of novel signal processing schemes, Mathematica modeling, experiment control and data acquisition, and finally sophisticated data analysis, error analysis, and searches for potential systematic errors that might skew the final result. Three undergraduate students were co-authors on the 2013 Physical Review article describing this work. The most recent of these, Nathan Schine '13 (now a Ph.D. student at U. Chicago), was selected as a finalist for the APS LeRoy Apker Award (a national undergraduate research prize) for his contributions to this experiment. In total, more than a dozen Williams students worked in this research lab during the funding period. Three were women, and three from under-representated minority groups. Half were first or second year students getting their first introduction to physics research at a crucial time when career decisions are beginning to be contemplated.
