Unlike the planets orbiting the sun, the electrons that whorl around the nucleus inside atoms are not allowed to do so at arbitrary distances. Quantum theory prescribes that only very specific average distances from the nucleus are allowed. This limitation gives rise to the specific colors of light that atoms respond to when hit with a laser beam. By tuning the laser precisely, one can find the specific colors that are in resonance with the orbiting electron, and thereby learn about the intricate details of the internal structure of the atom. While most atoms have many electrons, making the task of understanding the complicated pattern of orbits relatively difficult, hydrogen and helium have only one and two electrons, respectively, and hence are amenable to very precise theoretical prediction. By comparing these very precise predictions to similarly precise measurements, one can test whether or not the theory of "quantum electrodynamics" (the fundamental theory of light, atoms, and most of the rest of the ordinary world around us) is really correct, or only approximately so. The group supported by this grant plans to develop and deploy the laser sources and techniques that will allow improved tests of the quantum theory described above. If the theory is found to be correct at the new level of accuracy, the results of the experiments can then be used to determine the size of the nucleus in hydrogen and helium. A comparison of these sizes with what is known or predicted in nuclear physics will then provide a test of the scientific understanding of these simple nuclei. In this way, these experiments bridge the gap between the atom and the nucleus, and test the consistency of two regimes of physics which differ in length scale by a factor of 10,000. Laser sources and optical techniques are ubiquitous in science, technology and the economy, so training of students in this area provides valuable expertise, and has the potential to develop generally important laser sources and optical techniques for a wide range of applications.
This research develops laser technology and precision laser techniques. It applies them to the study of helium fine structure and the precise determination of hydrogen and helium nuclear sizes. Optical methods are developed which involve high-speed electro-optic modulation of a stabilized semiconductor laser, creating microwave tunable frequency side bands on the laser. These side bands can then be used to drive and determine the transitions frequencies of the triplet 2S -> 2P in helium 3 and 4, and 1S->2S in hydrogen and its isotopes. The physics of the laser interaction with hydrogen and helium atoms is carefully studied to obtain the unperturbed frequencies of studied atomic transitions. Results will be obtained with sufficient precision to provide important comparisons with theory. These include (1) the atomic theory and computation of electron-electron interactions in helium, the simplest multi-electron atom, and (2) the nuclear interactions and computations that predict the observed size of few-nucleon nuclei. In particular, isotopic shifts in these experimental results allow the charge radii of helium-3 and tritium to be determined and compared to predictions of few-nucleon theory and the underlying description of the nuclear force.
