Gravitational quantum states are traps for ultracold massive particles with gravity on top and a specularly reflecting mirror with a sharply changing surface potential on bottom. Energies of neutrons, hydrogen and antihydrogen atoms in gravitational states are of the order of ε0~0.6peV, and the characteristic size of wave functions is l0~5.9μm. Ultralow energies make this system very sensitive to any tiny interactions, and large sizes simplify the experimental techniques. Gravitational quantum states were discovered in experiments with ultracold neutrons in 2002, and since then they are actively used by several research groups (qBounce, Tokyo, GRANIT) at ILL, Grenoble. While repulsive neutron-nuclei optical potential of many materials totally reflect ultracold neutrons from surfaces, attractive van der Waals/Casimir-Polder potentials can also reflect ultracold atoms and molecules at surface due to quantum reflection. In contrast to the case of neutrons, nobody has ever observed gravitational states of atoms and antiatoms. Meantime, major motivations are growing up and prompting experimental efforts to observe gravitational states of atoms and antiatoms, and to improve to the maximum extend the precision in such experiments. Thus, gravitational states of antihydrogen in the GBAR project at CERN is the most precise method identified so far for the direct measurement of the gravitational acceleration of antimatter. Gravitational states and a related phenomenon of centrifugal quantum states of these particles is a sensitive method for the searches for extra short-range forces arising due to yet undiscovered light bosons or other phenomena beyond the Standard Model, manifestations of extra dimensions or dark matter. The techniques developed within these studies promise to help achieving ultralow energies of atoms thus providing unprecedented conditions for optical and hyperfine spectroscopy of with ultimate precision, which will be pursuit within the GRASIAN project.
[V.V. Nesvizhevsky, A.Yu. Voronin, Surprising Quantum Bounces, Imperial College Press, London, 2015]
Valery Nesvizhevsky
