The discovery, which represents yet another affirmation of the wave-particle duality inherent to quantum mechanics, is also the first published science to emerge from the £200 million second target station recently opened at the ISIS Neutron Source in the UK.
Newton suggested in the 17th century that a beam of light reflected at a glass-vacuum surface should undergo a small spatial shift. This arises from an interference effect, such that the light penetrates into the vacuum, “slides” along the interface, and then re-emerges and reflects back into the glass. The scale of the shift between the incident and reflected light is very small, and the effect was only confirmed experimentally in 1947, by the german physicists F Goos and H Hänschen.
Now, a team of scientists led by Rob Dalgliesh and Sean Langridge from the Science and Technology Facilities Council's ISIS facility and Victor de Haan from the Delft University of Technology (Netherlands) has proved experimentally that the so-called Goos-Hänschen shift also occurs with particles, such as neutrons. The result is therefore in perfect agreement with quantum mechanics, which predicts that, due to the quantized nature of energy, particles can behave as if they were waves, and vice versa.
As part of the second target station at ISIS, Dalgliesh, Langridge, and their colleagues in the Netherlands have designed an instrument called OffSpec, that can resolve length scales down to 10 nanometres. “We calculated that for neutrons, the Goos-Hänschen shift should range from nano- to micrometres, depending on the incident angle of the neutron beam”, explains Langridge. “Our objective was therefore to measure this with OffSpec.”
The trick used by the researchers was to exploit the fact that a neutron has a magnetic moment which can be described in terms of a wave function that has both “up-spin” and “down-spin” components. The Goos-Hänschen shift is different for the up and down wave functions, meaning that the polarization of a beam of neutrons changes upon reflection from a surface. The outcome is that during reflection the up and down spin states become split in space and in time.
Langridge points out that only an instrument with a high flux of neutrons with a specific wave length, such as OffSpec, can measure the change in polarization of a neutron beam. “Although our results demonstrate that neutrons behave exactly in the same way of light when they are reflected, they are also a showcase for the kind of things OffSpec can do in the future”.