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Le Journal de Physique Colloques

NEUTRON WAVEPACKETS AND LONGITUDINAL COHERENCE

A. G. Klein, G. I. Opat

Published Mar 1, 1984• 0 citations
Publisher

Abstract

The concept of wavepackets is widely used in visualizing the behavior of particles in optical experiments. The length of wavepackets, however, is shown not to be synonymous with coherence length as measured by interferometry. Is the length of wavepackets accessible to experimental measurement ? A possible approach via time-resolved interferometry is proposed.

DOI
10.1051/jphyscol:1984339

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The concept of wavepackets is widely used in visualizing the behavior of particles in optical experiments. The length of wavepackets, however, is shown not to be synonymous with coherence length as measured by interferometry. Is the length of wavepackets accessible to experimental measurement ? A possible approach via time-resolved interferometry is proposed.
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We point out that many wavepacket discussions for the coherence properties of particle beams are unnecessary since they deal with stationary sources; and when the problem is stationary, essentially all information is in the energy spectrum. This recognition allows a simple answer to a number of long-debated points, usually framed in terms of the``length of the wavepacket.'' In particular we discuss neutrino oscillations, and some issues in neutron physics. The question as to whether two simple beams with the same energy spectrum are distinguishable is answered negatively for stationary situations. The question as to whether neutrino oscillations should be thought of as taking place between states of the same energy or the same momentum is answered in favor of energy for stationary situations. The consequences for proposals involving the 7Be{}^{7}\mathrm{Be}7Be neutrino line of the Sun, the observation of oscillations in supernova neutrinos, and wavepacket studies with neutrons are briefly discussed, as well as the connection with the coherence notions of quantum optics.
The study and use of macroscopic quantum coherence requires long coherence lengths. Here we describe an approach to measuring the vertical coherence length in neutron interferometry, along with improvements to the NIST interferometer that led to a measured coherence length of 790 A. The measurement is based on introducing a path separation and measuring the loss in contrast as this separation is increased. The measured coherence length is consistent with the momentum distribution of the neutron beam. Finally, we demonstrate that the loss in contrast with beam displacement in one leg of the interferometer can be recovered by introducing a corresponding displacement in the second leg.
Neutrons are important probes of mater and quantum physics. They are particularly powerful at characterizing magnetic structures. In order to extend the applications of neutron physics as a quantum probes/sensors we have developed methods based on Quantum Information Processing for preparing structured waves such as orbital and spin-orbit states of neutrons. Such states may be prepared for the beam as a whole, or with reference to the coherence length of the neutron wavepacket, which is much smaller than the width of useful beams. I will describe experiments to prepare and characterize neutron beams with specific orbital and spin-orbit structure based on neutron interferometry and will show how we can apply these methods to other type of probes such as light.
Abstract Coherence phenomena play an important role in any kind of interferometry (Gabor 1956; Glauber 1963; Mandel and Wolf 1965, 1995; Born and Wolf 1975; Walls and Milburn 1994; Mandel and Wolf 1995). In this chapter we summarize some known results, add some new ones obtained with neutrons, and analyse them in terms of general quantum optics, which can be applied to photon and matter waves as well. Indeed, neutrons have many well-known particle properties, but in interference experiments they behave like wave fields, which provides the connection to the quantum optical terminology. Coherence appears as a system property (neutron + apparatus) which persists as long as it does not become destroyed by statistical or dissipative effects.
Abstract Among the various areas of neutron optical research, neutron interferometry has provided some of the most challenging and spectacular developments. In most cases, separated coherent beams are used which are produced either by wavefront division (Young’s type) or by amplitude division (Mach− Zehnder type). These beams are subsequently coherently superposed after passing through regions of space where the neutron wavefunction is modified in phase and amplitude by various interactions: nuclear, magnetic, electromagnetic, gravitational, or by geometry and topology (as in Aharonov−Bohm-type experiments). Various types of neutron interferometers that have been tested are shown in Fig. 2.1. The first neutron interferometer was based upon wavefront division and biprism deflection, but it had several constraints for interferometric applications mainly due to the small beam separation (100 μm) (Maier-Leibnitz and Springer 1962). Nevertheless, this method gained further interest for diffraction experiments from macroscopic objects and for the detection of weak interaction effects where the long flight paths (10 m) enhance the sensitivity for the measurements of these effects (Gähler et al. 1980; Klein et al. 1981a). The perfect-crystal interferometer (Rauch et al. 1974) provides widely separated coherent beams and has become a standard method for advanced neutron optical investigations. The development of diffraction and phase grating interferometers extended the Mach-Zehnder interferometer method to very slow neutrons (loffe et al. 1985; Gruber et al. 1989). The superposition of spin-up and spin-down states in the longitudinal direction provides the basis for Larmor and Ramsey interferometry, where no lateral beam splitting is necessary. The well-known spin-echo systems (Mezei 1972) and zero-field spin-echo systems (Gähler and Golub 1987; Dubbers et al. 1989) are examples of such interferometers, which also play an important role in atom interferometry (Bord é 1989; Kasevich and Chu 1991; Riehle et al. 1991).