On neutron holography, neutron interferometry and neutron orbital angular momentum

Abstract: A neutron Laue crystal interferometer has been reported by Saranac et al . to demonstrate neutron holography of a spiral phase plate. Using its two coherent beams as the object and reference beams, the resulting interference pattern was interpreted as a hologram. This interference pattern was then reported to reconstruct neutron beams with various intrinsic orbital angular momenta. There are serious doubts about the method for generating neutron orbital angular momentum with a crystal interferometer. Due to the extremely different lateral coherence lengths in the interferometer, one should expect the pattern described as a hologram to be asymmetric. In addition, a neutron crystal interferometer always produces a Moire image where the beams are combined in the final crystal, that appears as a one-dimensional enlargement of the interference pattern. We present computer simulations showing that the images presented as holograms can be computed as conventional interference patterns assuming the phase shifts of ordinary neutrons passing through objects located in one or the other beam path of the interferometer. Additionally, we discuss the complications of using a crystal interferometer for holography, while raising the question of whether the intrinsic orbital angular momentum states of neutrons are necessary or sufficient to explain the recorded images.