Measurement of the Gravitostatic Aharonov-Bohn Effect

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Of the fundamental forces, gravity is probably the one most familiar in everyday life, yet its physics is the least well understood. For one, it does not fit into the standard model of particle physics, which is based on quantum mechanics. In addition, the action of gravity in quantum mechanics has been relatively elusive to experimental measurement. One of the most important challenges in physics is thus to find new ways to investigate how gravity affects quantum systems. The group will give a demonstration of the so-called Aharonov-Bohm effect (AB effect) for gravity. In classical physics, the interaction between two objects can be described equivalently based on forces or on potential energies. Aharonov and Bohm, however, pointed out that in quantum mechanics, two objects can influence each other even if there are zero forces between them. This has been verified experimentally for electromagnetic forces and potentials. The group will construct a specialized apparatus to investigate whether there is an analogous Aharonov-Bohm effect for gravity. This will demonstrate that gravity affects a quantum system even when it does not cause any classical force; that knowledge of the classical gravitational force acting locally on a particle is not sufficient to predict the particle's quantum-mechanical behavior; and help develop a new way of measuring Newton's gravitational constant G. Paraphrasing R. P. Feynman, for a long time it was believed that potentials were not "real", since they could be replaced by force fields in the description of any observable phenomenon then known. However, the AB effect has established that potentials are real, and forces are slowly disappearing from the modern expression of physical law. The group is collaborating with industrial partners to apply the techniques developed in this work in the production of advanced inertial sensors for improved navigation and positioning.The experiment will be based on laser-cooled cesium atoms held in an optical lattice, inside a special arrangement of two spherical masses that produce a "W"-shaped gravitational potential. The atoms will be at extrema of the gravitational potential, so that there will be no net force between the spherical masses and the atoms. Nevertheless, the masses should cause a relative phase shift of the atomic matter waves. The group will experimentally demonstrate the distinguishing features of the AB effect, such as nonlocality and nondispersiveness. The optical lattice will be a standing wave inside a Fabry-Perot cavity, to control the optical wave-fronts and provide laser intensity enhancement.