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Acta Physica Polonica A

Cold Neutron Interferometry Using Multilayer Neutron Mirrors

Takeshi Kawai

Published Jul 1, 1999• 0 citations
Publisher

Abstract

The object of this report is to review the recent results and some perspectives on cold neutron interferometry using multilayer neutron mirrors developed in KUR.The purposes of our research are to develop new experimental techniques, new devices for controlling neutron spin states for opening up a new world of physics.A multilayer neutron polarizes working under a very low external magnetic field (low field magnetic mirror), Funahashi's interferometer, a cold neutron spin echo instrument, a multilayer neutron spill splitter and a cold neutron pulser were developed.The equiv- alency of the quantum precession induced by the effective gap layer of the multilayer neutron spin splitter and Larmor precession of neutron spin in a magnetic field is established.This idea allows us to develop a cold neutron phase-spin-echo interferometer (a neutron spin interferometer).The neutron spin interferometer and a cold neutron pulser enabled us to carry out the delayed choice experiment for answering the question "how the neutron wave propagates after being split by the semitransparent mirror (wave splitter)".• These matters are reviewed in this paper.•

DOI
10.12693/aphyspola.96.33

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The object of this report is to review the recent results and some perspectives on cold neutron interferometry using multilayer neutron mirrors developed in KUR.The purposes of our research are to develop new experimental techniques, new devices for controlling neutron spin states for opening up a new world of physics.A multilayer neutron polarizes working under a very low external magnetic field (low field magnetic mirror), Funahashi's interferometer, a cold neutron spin echo instrument, a multilayer neutron spill splitter and a cold neutron pulser were developed.The equiv- alency of the quantum precession induced by the effective gap layer of the multilayer neutron spin splitter and Larmor precession of neutron spin in a magnetic field is established.This idea allows us to develop a cold neutron phase-spin-echo interferometer (a neutron spin interferometer).The neutron spin interferometer and a cold neutron pulser enabled us to carry out the delayed choice experiment for answering the question "how the neutron wave propagates after being split by the semitransparent mirror (wave splitter)".• These matters are reviewed in this paper.•
We demonstrate a method of inducing neutron spin rotation in a field-free region of space. A multilayer mirror (``spin splitter'') is used as an optically active element, providing a longer path length for one spin eigenstate of a polarized neutron than for the other. This produces a relative phase shift between the two eigenstates, which is interpreted, quantum mechanically, as precession of the neutron spin. We show that the spin precession of our technique is equivalent to Larmor precession in a magnetic field, though our method has no such classical analog and occurs on shorter (e.g., 20 nm) length scales. We also demonstrate a ``phase spin-echo'' interferometer based on a pair of multilayer spin splitters.
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).
In this paper, we review how neutron–mirror-neutron mixing can induce a geometric phase for ordinary neutrons and how this phase can be accessed with a two-arm interferometer. By choosing different magnetic fields and suitable arm lengths, the dynamical phase difference can be canceled, so that the observed phase is directly related to the mixing. We summarize the main formulas and discuss representative regions of the neutron–mirror-neutron parameter space that can be explored with cold-neutron interferometry [A. Capolupo, G. Pisacane, A. Quaranta and F. Romeo, Phys. Dark Univ. 46 (2024) 101688].
We are developing large‐dimensional cold‐neutron interferometers with multilayer mirrors in order to investigate small interactions. In particular Jamin type interferometers composed of wide‐gapped “BSE”s, which divide the beam completely, can realize the precision measurement of topological Aharonov‐Casher effect. We have made a prototype with 200 μm gapped BSEs and confirmed the spatial separation of its two paths at monochromatic cold‐neutron beamline MINE2 on JRR‐3M reactor in JAEA.
Neutron resonance spin-echo (NRSE) phenomena are investigated experimentally and theoretically. In our experiments, spin precession produced in a classical manner and by neutron resonance is combined as two arms of a spin-echo machine. A magnetic-field scan in the classical SE coil reveals a spin-echo signal of the precession produced by the NRSE arm. The neutron spin-flip probability $\ensuremath{\rho}$ in the resonance coils turns out to be a key parameter of the NRSE arm. The limiting cases $\ensuremath{\rho}=0$ and $\ensuremath{\rho}=1$ lead, respectively, to Larmor precession with phase ${\ensuremath{\varphi}}_{1}$ in the static magnetic fields of the NR flippers or to NRSE precession with ${\ensuremath{\varphi}}_{2}.$ The case $0<\ensuremath{\rho}<1$ produces quantum interference resulting in additional echoes with phases ${\ensuremath{\varphi}}_{3}=({\ensuremath{\varphi}}_{1}+{\ensuremath{\varphi}}_{2})/2$ and ${\ensuremath{\varphi}}_{4}=\ifmmode\pm\else\textpm\fi{}({\ensuremath{\varphi}}_{1}\ensuremath{-}{\ensuremath{\varphi}}_{2})/2.$ The amplitude of each pattern depends on the spin-flip probability $\ensuremath{\rho}$ and the initial polarization. These experiments demonstrate explicitly the quantum-mechanical principle of linear spin state superposition of neutron particle waves, and the interference as a result of that.