Neutron Interferometry: Lessons in Experimental Quantum Mechanics
DOI: 10.1063/1.1496381
The availability of copious quantities of thermalized neutrons makes them an ideal probe in condensed matter physics and materials research. This same abundance makes them the particle of choice for many fundamental physics investigations. A prime example is neutron interferometry, which is a technique developed to investigate a wide variety of fundamental aspects of quantum theory.
Helmut Rauch and Samuel Werner have been pioneers in the field of neutron interferometry since the first demonstration of a neutron interferometer in 1974; their book, Neutron Interferometry: Lessons in Experimental Quantum Mechanics , is both a very readable introduction to the subject and a comprehensive and up-to-date review of this elegant experimental technique. It is written for the advanced graduate student and researcher and serves as a reference text for the field of neutron interferometry.
Interferometry is a technique familiar to all physicists and can be carried out with any wave phenomenon. An incident beam of particles with wavelength λ is split and then recombined, forming an interference pattern that is sensitive to any change in the effective path length of one or both legs of the interferometer. Changing this “optical” path length in a controlled manner allows the experimenter to probe the perturbing interaction with extraordinary precision.
In contrast to the familiar optical interferometers, which employ wavelengths ≈10−6 meters, for neutron interferometry the de Broglie wavelength is about 10−10 m. The neutron interferometer uses Bragg reflection from perfect silicon single crystals, in place of mirrors, both to split the beam and recombine it. Neutrons are advantageous in that they are sensitive to the four basic interactions—strong, weak, electromagnetic, and gravitational—which makes the neutron interferometer a particularly versatile tool for testing fundamental physics concepts. Indeed, just the existence of the neutron interferometer is a striking example of the wave–particle duality of quantum mechanics.
The first half of Neutron Interferometry introduces the basic aspects of neutron interferometers and the interactions of neutrons with matter. It is a very readable exposition, which, by necessity, includes the detailed quantum mechanical mathematics to elucidate fully the fundamental operation of neutron interferometers and the utility of the technique. This material can be compared with the compendium of articles edited by Ulrich Bonse and Rauch, Neutron Interferometry (Oxford U. Press, 1979). The present text brings together in a coherent description most of the material in this earlier collection; it also, of course, includes developments in the field in the intervening 20 years.
The second part of the book describes some of the benchmark experiments of neutron interferometry. For example, rotating a classical vector by 2 π restores the original state, while, quantum mechanically, the rotation of the spin of an S = 1/2 fermion particle is expected to change the sign of wave-function, and the spin must be rotated by 4π to return to its initial value (Ψ(0) = — Ψ (2π) = Ψ (4π)). This 4π spinor symmetry of fermions was for many years thought to be an unobservable nuance of quantum mechanics— until it was demonstrated experimentally, with a neutron interferometer, by varying a magnetic field in one leg and observing the change in the interference pattern. The neutron interferometer has been used in an analogous manner to examine a wide variety of topological and geometrical effects on the phase of the neutron. Examples include the Aharonov–Casher effect (vector Aharonov–Bohm effect) and gravitationally induced quantum interference.
One of the interesting aspects of the Rauch–Werner work is the willingness of the authors to gaze into the future. They discuss possible applications of interferometry in materials science, an application that is just in its infancy, and they include an intriguing chapter on “forthcoming and more speculative experiments.” This chapter incorporates tests for nonlinear terms in the Schrödinger equation, quaternions in quantum mechanics, delayed choice experiments, non-Newtonian gravity effects, and a host of other fascinating possibilities. The only shortcoming in Rauch and Werner’s text is the very short, two-page index. Interferometry is a complex and advanced subject, and a more complete index would have been useful, particularly in a reference text for people who are not full-time practitioners in the field.
It is clear that the field of neutron interferometry will continue to be vital and exciting for many years to come and that Rauch and Werner’s Neutron Interferometry will become the standard text for the field.
More about the Authors
Jeffrey W. Lynn. National Institute of Standards and Technology, Gaithersburg, Maryland, US .
