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Physical Review Letters

Spectral filtering in neutron interferometry

S. A. Werner, R. Clothier, Helmut Kaiser, H. Rauch, H. Wölwitsch

Published Aug 5, 1991• 33 citations
Publisher

Abstract

We have demonstrated experimentally that the contrast lost, due to placing slabs of a material having a neutron optical potential in one leg of a neutron interferometer, can be restored by spectral filtering of the beams leaving the interferometer immediately before detection. The non-Gaussian spectral distribution of the neutrons traversing the interferometer in our experiment leads to an unexpected phase reversal of the interferograms.

DOI
10.1103/physrevlett.67.683

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Physical Review A•Jul 1, 1993•4 citations
We have demonstrated experimentally that the contrast lost, due to placing slabs of a material having a neutron optical potential in one leg of a neutron interferometer, can be restored by spectral filtering of the beams leaving the interferometer immediately before detection. The non-Gaussian spectral distribution of the neutrons traversing the interferometer in our experiment leads to an unexpected phase reversal of the interferograms.
The coherence length of a neutron wave packet depends on the spectral width of the distribution of wave vectors that make up the packet. The coherent overlap of the wave packets traversing the two beam paths in a perfect-silicon-crystal neutron interferometer (NI) is altered by placing a material with a neutron-nuclear optical potential in one of the beam paths in the NI. If the optical potential is positive, it causes a delay of the wave packet and a loss of fringe visibility. By use of an analyzer crystal, we narrow the spectral distribution after the mixing and interference has occurred in the last crystal slab of the NI. This increases the coherence length and restores some of the fringe visibility.
The coherent overlap of the waves traversing the two legs of a perfect-crystal neutron interferometer is altered by a material with a neutron-nuclear optical potential. The fringe visibility is limited by the longitudinal coherence length. The loss of interference contrast caused by a slab of material of positive optical potential can be restored by placing another slab of material with a negative optical potential in the same leg of the interferometer. This paper describes the observation and detailed characterization of this quantum-mechanical phenomenon, which we call the phase-echo effect.
In the experiment described in this paper, a chopper serves as a source of neutron intensity pulses which pass through a perfect Si-crystal neutron interferometer (NI), where they are split coherently into two parts that recombine and interfere. As the occupation number of a single pulse is much smaller than unity, the results very clearly demonstrate the single-particle interference phenomena of neutron interferometry experiments. While traveling the pulses spread, with faster (slower) neutrons tending toward the leading (trailing) edge. The coherence length of the neutrons depends on the spectral width of their wavelength distribution. When a material with a neutron-nuclear optical potential is placed in one beam path in the NI, there is a loss of fringe visibility (contrast), which depends on the coherence length. By using time-of-flight techniques, we divide the pulse into time segments. Within a given time slice, the wavelength spectrum is narrower than in the pulse as a whole. As a result, contrast remains in the time slices even when it disappears in the overall pulse. We also observe an additional contrast modulation due to the overlap of neighboring pulses.