A surprising prediction about the quantum properties of neutron beams made in the late 1980s has been confirmed in experiments. More than a decade ago Jean-Marc Levy-Leblond and Daniel Greenberger had independently predicted that a neutron beam passing through a slit would experience a phase shift in its direction of motion. Now Helmut Rauch and co-workers at the Atomic Institute of the Austrian Universities in Vienna and the Institut Laue-Langevin (ILL) in Grenoble have measured this confinement induced phase shift in an experiment (H Rauch et al 2002 Nature 417 630).
When a neutron beam passes through a slit its transverse motion is quantized, just like a particle in a box. Indeed, for the experimental set-up used by Rauch and co-workers, there are 360 bound states in the potential created by the slit. Levy-Leblond and Greenberger predicted that this quantization in the transverse direction would lead to a phase shift in the longitudinal direction that could be detected with a neutron interferometer. This is what the Vienna-Grenoble team has done.
Rauch and co-workers actually used a silicon multi-slit system consisting of 186 slits – each 22.1 microns wide – to increase the intensity in their experiment. A thermal beam of neutrons from the ILL reactor was converted into a beam with a well-defined neutron energy and this beam was then split into two components. One component passed through the slit system, while the other did not. The two beams were then recombined and the neutron signal was measured as a function of the path difference between the two beams. The measured value of the shift was 2.8 degrees, which was in good agreement with the theoretical prediction of 2.5 degrees.
The phase shift arises mostly from neutrons whose classical trajectories do not touch the walls of the slits, and is therefore another example of the non-local nature of quantum mechanics. Earlier this year another team at the ILL observed quantum motion in a neutron beam in the gravitational potential of the Earth – the first time that quantum motion had been observed in a gravitational potential.
