PM: Precision Laser Spectroscopy of 2S-nS Two-Photon Transitions in Hydrogen

Colorado State University, Fort Collins CO

Investigators

Abstract

For this project, a laser will be used to probe the internal quantum states of atomic hydrogen. Hydrogen is the simplest atom and is comprised of only one electron and one proton. Due to this simplicity, the theory describing hydrogen is very well-developed and precise measurements in hydrogen can provide a stringent test of our best physical theories and determine key fundamental constants of nature. One constant that can be determined through hydrogen measurements is the proton size, which can also be measured at high-energy physics facilities where electrons are scattered off of protons, or through measurements of an exotic form of hydrogen where the electron is replaced by a heavier fundamental particle called a muon. Determinations of the proton size in these different physical systems have produced inconsistencies, which may be indicating a problem with our current theories, or the need for additional theories. This project will help bring clarity by providing measurements with an order of magnitude less uncertainty over previous measurements of the same hydrogen states. The planned experiments will train students in the fields of atomic, molecular, and optical physics, precision laser science, and precision measurement. Ultimately, this project could provide more reliable values for our fundamental constants, establish consistency in the physical constants derived from different physical systems and within different scientific disciplines, or possibly provide indications of new physical laws. The goal of this project is to perform high-precision laser spectroscopy of atomic hydrogen. The specific transitions to be measured will be the hydrogen 2S-nS two-photon transitions (with n between 8 and 12). In conjunction with well-developed hydrogen theory, these measurements can be used to extract the Rydberg constant and proton charge radius. Previous determinations of these constants in different physical systems – such as other hydrogen transitions, measurements in muonic hydrogen, and electron scattering – have produced inconsistencies which may indicate new physics. The spectroscopy will be performed on a cryogenic and velocity-characterized hydrogen beam, and the spectroscopy laser will be a cavity-enhanced continuous-wave Ti:sapphire laser referenced to a coherent optical frequency comb. In previous measurements, the AC Stark shift broadened and distorted spectroscopic resonances which introduced systematic uncertainty. However, for these experiments, an additional laser with a wavelength of approximately 650 nm will be used to cancel the AC Stark shift. This cancellation will allow for the recovery of narrow Lorentzian resonances with widths approaching the natural linewidths – between 50 kHz and 144 kHz – and absolute frequency measurements with relative uncertainties of less than 1 part in a trillion (twelve digits of accuracy), which represents a one order of magnitude decrease in uncertainty over previous measurements of those same transitions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.