The program in atomic physics involves work with simple atomic and molecular systems in the gas phase, at surfaces, and in solids. The inherent precision of measurements on simple atomic and molecular systems is used in studies of fundamental physics as well as for certain applications. Experimental work frequently involves the use of Nd:YAG lasers, dye lasers, diode lasers, Ti:sapphire lasers, argon and krypton ion lasers, optical, ultraviolet and infrared spectrometers, microwave and radiofrequency spectrometers, signal processing equipment, and computers and their interfacing to apparatus. Students often need a detailed theoretical understanding of certain aspects of quantum mechanics, group theory, electricity and magnetism, nuclear physics, surface physics, physical chemistry, fluid dynamics, and plasma physics to complete their dissertation work.
An important area of active research is the study of spin interactions of atoms and nuclei. Princeton has historically been, and continues to be, a world center for such activities. For instance, optical pumping is used to produce high polarizations in vapors of alkali-metal atoms. Hyperpolarized Helium Lung Image Through spin-exchange collisions, the polarization of the alkali-metal atoms can be transferred to a variety of noble-gas nuclei. The use of "optical pumping spin-exchange" makes it possible to polarize the nuclei of large quantities of gases such as He-3 and Xe-129 for a variety of applications. One such application is a new technique for doing magnetic resonance imaging (MRI). Polarized noble gases are introduced into the lungs of an animal and subsequently imaged using fairly standard MRI techniques. The use of laser-polarized noble gases results in enormous increases in signal strength, thus opening many new possibilities for medical imaging. Another application is the production of polarized nuclear targets for accelerator-based experiments. Many of the innovations that make laser-polarized gas targets a practical possibility were developed at Princeton.
On the more fundamental side, the spin interactions of atoms offer a huge and rich area for study. The interaction of polarized light with atoms, of polarized atoms with each other, and of polarized atoms with surfaces, are all active areas of research. Important questions include identifying the mechanisms that cause spin relaxation, as well as the means by which polarization can flow from one system to another. Research topics include the spin interactions of atoms that are in the solid state, such as xenon ice and alkali-metal hydrides. In much of this work, laser techniques are used in combination with nuclear magnetic resonance to produce powerful new experimental probes.
Spin-polarized atoms are also used to address fundamental questions in particle physics. Precision measurements of interactions between spin-polarized atoms and external fields can reveal the properties of elementary particle interactions. Time reversal symmetry, CP and CPT symmetry, and Lorentz invariance are being tested in such experiments. For example, measurements of the precession frequency of Xe-129 spins in a strong electric field determine the electric dipole moment of Xe-129, which is sensitive to CP violation beyond the Standard Model needed to explain the asymmetry between matter and anti-matter in the Universe.