Neutron whispering gallery
Dr Valery Nesvizhevsky, European Centre for Neutron Research, Institut Laue-Langevin
The "whispering gallery" effect has been known since ancient times for sound waves in air, later in water and more recently for a broad range of electromagnetic waves: radio, optics, Roentgen and so on. It consists of wave localization near a curved reflecting surface and is expected for waves of various natures, for instance, for atoms and neutrons. For matter waves, it would include a new feature: a massive particle would be settled in quantum states, with parameters depending on its mass. In 2010, we observed the quantum whispering gallery effect for cold neutrons and since then continue increasing the precision in these experiments. This phenomenon provides an example of an exactly solvable problem analogous to the "‘quantum bouncer"; it is complementary to gravitational quantum states of neutrons. These two phenomena provide a direct demonstration of weak equivalence principle for a massive particle in a pure quantum state. Deeply bound whispering-gallery states are long-living and weakly sensitive to surface potential; highly excited states are short-living and very sensitive to the wall surface potential shape. Therefore, they are a promising tool for studying fundamental neutron-matter interactions, quantum neutron optics and surface physics effects.
[V.V. Nesvizhevsky, A.Yu. Voronin, Surprising Quantum Bounces, Imperial College Press, London, 2015]