Improving the Experimental Characterization of Dipole Matrix Elements in Rubidium

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An atom consists of electrons bound to a nucleus. This is sometimes depicted with the electrons orbiting the nucleus like planets orbit the sun, but 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 a probabilistic way. In order to understand an atom’s properties, it is necessary to know the nature of this distribution. Unfortunately, the distribution is hard to observe directly with high precision. Typically, the best approach is to measure properties of the atom that depend on the electrons and use these values to constrain the distribution. One useful set of properties are the dipole matrix elements, which help describe how the electrons move when the atom is placed in an electric field. Even the dipole matrix elements cannot be measured directly, but they can be reliably inferred from several types of experiments. A particularly precise method is called tune-out wavelength spectroscopy. In this project, tune-out measurements will be used to obtain improved values for several matrix elements of the rubidium atom. To give one example of why these measurements are important, consider the question of whether an atom has “handedness,” like a right-handed or left-handed glove. Theories and measurements in nuclear physics indicate that the atomic nucleus does have 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 addition to these scientific results, the project will provide an important training opportunity for both undergraduate and graduate students in physics. The techniques used in this project are applicable to other important research areas such as quantum computing and quantum communication, as well as to nationally important technologies such as lasers and quantum sensing. Participants in this project will be well prepared to contribute in these areas.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. There are multiple wavelengths at which tuneouts occur, and the wavelengths also depend on the polarization of the light. This project will result in a precise measurement of the polarization dependence for Rb atoms at a wavelength near 790 nm, and also precise measurements near 420 nm. By combining these measurements, it will be possible to determine a number of small but important terms, such as the total contribution of the high-lying atomic states, and also the impact of interactions between the valence electron and the atomic core. In addition, spectroscopic measurements will be used to measure the matrix elements from the 5P excited state of Rb to high-lying states. Although this will not be a high-precision measurement, the results will be useful for interpreting experiments like the handedness measurement described above. Finally, the possibility of using Rb atoms for a handedness (or parity violation) measurement will be explored. Although the effect is smaller in Rb than in some other atoms, Rb atoms are very convenient to use with modern cooling and trapping techniques. It may be that the benefits of these techniques make a competitive option for improving precision, and the measurements would be able to take direct advantage of the improved matrix elements which will be obtained in the project.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.