Precision Measurement of Atomic Dipole Matrix Elements Using Tune-Out Wavelength Spectroscopy

University Of Virginia Main Campus, Charlottesville VA

Investigators

Abstract

Although an atom is sometimes depicted with the electrons orbiting the nucleus like planets orbit the sun, in reality motion at the atomic scale is governed by the laws of quantum mechanics. These say that even though the electron is a point-like particle, it is impossible to say exactly where an electron is at any given time. Instead, the location of an electron is spread out around the nucleus. In order to understand an atom's properties it is necessary to know how all its electrons are spread out. One useful set of properties that measure how electrons are distributed are the "dipole matrix elements" (numerical values that arise from particular mathematical constructs which help describe how the electron distributions change when the energy of the atom is changed). Even the dipole matrix elements cannot be measured directly, but they can be reliably inferred from several types of experiments. Recently, a new way to obtain dipole matrix elements more precisely has been demonstrated, called tune-out wavelength spectroscopy. For this project, tune-out wavelength measurements will be used to obtain improved values for several matrix elements of the rubidium atom. One example of why these measurements are important relates to whether an atom has "handedness," like a right-handed or left-handed glove. Theories and measurements in nuclear physics indicate that the atomic nucleus has a handedness. The role of this handedness in nuclear physics is very important for understanding how matter was created in the early universe, and why the universe looks the way it does now. It can be studied in large particle accelerators, but doing so is difficult and expensive. In an atom, a slight amount of the handedness is transmitted from the nucleus to the electrons. Although the effect is small, the atomic handedness can be measured very precisely and this has been useful for improving nuclear physics theories. However, relating the atomic measurement to nuclear physics requires accurate dipole matrix elements. The improved values that will be provided by this project will be useful for this purpose, and in this way help improve our understanding of why the universe is as it is. In tune-out wavelength spectroscopy, the atom is illuminated by a laser beam that is tuned far from any atomic resonances. In most cases, this causes the energy of the atom to change. The energy can either increase or decrease, depending on the wavelength of the light. At specific wavelengths, however, the energy change is exactly zero. The value of these wavelengths depends on the dipole matrix elements of the atom, and by measuring the wavelength accurately the ratios of various matrix elements can be determined with high precision. The method used in this project to determine the tune-out wavelength is atom interferometry. Here the overall wave function of the atom is split into two branches that separate in space. One branch passes through the laser, and any resulting energy change results in a phase shift of the wave function. When the two branches are later recombined, this phase shift can be detected. The tune-out wavelength is determined by adjusting the laser to make the phase shift zero. A particular focus of this project will be to observe how the tune-out wavelength depends on the polarization of the laser light. By combining polarization-dependent measurements at a few different tune-out wavelengths, it is possible to accurately extract several groups of small matrix elements that are otherwise very challenging to measure or calculate. These small matrix elements are nonetheless important, as they represent dominant uncertainties in measurements of atomic parity violation and other atomic properties like the dc electric polarizability.