Institut Laue-Langevin

Neutron whispering galleries

A recent ILL experiment on centrifugal quantum states has just been highlighted in the journal Nature Physics. The experiments succeeded in demonstrating for the very first time the weak equivalence principle of general relativity for a massive particle in a pure quantum state.

In fact, the experiment, performed on the D17 reflectometer, illustrates the quantum whispering-gallery effect for cold neutrons. This is another example of an exactly solvable problem analogous to the ‘quantum bouncer’; it complements ILL's recent demonstration of gravitationally bound neutron quantum states. In this case, however, centrifugal force replaces gravity.
The "whispering gallery" effect was already known in ancient times for air-borne, and then later water-borne, sound waves. It has more recently been extended to a broad range of electromagnetic waves: radio, optical and Roentgen waves for instance. The effect consists of wave localization near a curved reflecting surface and can be expected for waves of different types – including atoms and neutrons. For waves of matter, it would include a new feature: a massive particle would be settled in quantum states, with parameters depending on its mass.

Deeply bound whispering-gallery states are long-lived and weakly sensitive to surface potential; highly excited states are short-lived and very sensitive to the wall potential shape. They are therefore a promising tool for studying fundamental neutron–matter interactions, quantum neutron optics and surface physics effects.

This is part of the GRANIT programme and will be continued at ILL in the years to come. A dedicated workshop GRANIT-2010 took place in Les Houches on 14-19 February 2010. See the workshop website.

Ref.: V.V. Nesvizhevsky, A.Yu. Voronin, R. Cubitt and K.V. Protasov (2009).
Nature physics, December 2009 - Vol 5 No 12

Contact: nesvizh(at)ill.eu