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Physical Review A

Restoration of neutron interferometer contrast by post-interference filtering

J. W. G. Wignall

Published Feb 1, 1993• 1 citation
Publisher

Abstract

It is argued that the results of a recent neutron interference experiment [Kaiser et al., Phys. Rev. A 45, 31 (1992)], in which contrast lost as a result of difference in effective path lengths is restored by wavelength filtering after recombination, cannot be construed as evidence against the notion that a neutron is a physically real wave packet.

DOI
10.1103/physreva.47.1500

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An experiment with nested Mach-Zehnder interferometer [Phys. Rev. Lett. 111, 240402 (2013)] has been recently implemented with neutrons [Phys. Rev. A 97, 052111 (2018)]. Which-path information has been extracted from faint traces the neutrons left, providing operational meaning to "the particle's path". The authors of the neutron interference experiment criticised the conclusions obtained by the authors of the optical experiment. I refute the criticism and argue that the results of the neutron interference experiment actually support the surprising picture of the past of the particle in the nested Mach-Zehnder interferometer.

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It is argued that the results of a recent neutron interference experiment [Kaiser et al., Phys. Rev. A 45, 31 (1992)], in which contrast lost as a result of difference in effective path lengths is restored by wavelength filtering after recombination, cannot be construed as evidence against the notion that a neutron is a physically real wave packet.
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.
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.
An experiment with nested Mach-Zehnder interferometer [Phys. Rev. Lett. 111, 240402 (2013)] has been recently implemented with neutrons [Phys. Rev. A 97, 052111 (2018)]. Which-path information has been extracted from faint traces the neutrons left, providing operational meaning to "the particle's path". The authors of the neutron interference experiment criticised the conclusions obtained by the authors of the optical experiment. I refute the criticism and argue that the results of the neutron interference experiment actually support the surprising picture of the past of the particle in the nested Mach-Zehnder interferometer.