Photons, Orbital Angular Momentum, and Neutrons

Abstract

A traveling wave or wave packet may possess orbital angular momentum (OAM) in the form of a phase vortex about its propagation axis. These OAM states are a general wave property and exist in single‐particle wave packets, in a beam of unstructured wave packets of particles or in a mixture. OAM states of photons were first created almost 30 years ago. Recently work has been done to create these states with massive particles: electrons and neutrons. OAM waves can be generated by passing an unstructured wave through a vortex phase plate. This technique is reliant on each particle adequately sampling the area of the vortex plate. While laser sources provide sufficient transverse coherence, beams of massive particles are more challenging. If incident wave packets do not interrogate the discontinuity at the center of the vortex phase plate, the probability of detecting a single particle having a unit of OAM about the packet center decreases substantially and the expectation value of the OAM about that center vanishes. Herein, using laser light, we restrict the transverse size of a beam to either illuminate the discontinuity or not and consider the implications for producing OAM in individual neutrons.

Promises and challenges of high-energy vortex states collisions

“…As a result, each individual neutron passing through the interferometer probed only a small off-centered patch of the phase plate and did not exhibit a well-formed phase vortex. These observations raised doubts as to whether the results of this experiment were at all indicative of vortex neutron generation [247,248].…”

Computerized Simulation of 2-Dimensional Phase Contrast Images Using Spiral Phase Plates in Neutron Interferometry

“…Concerning a SPP-induced neutron OAM (n-OAM), experiments with laser light using a SPP have shown that incident wave packets must be able to detect the discontinuity at the center of the vortex phase plate, otherwise the probability of detecting a single particle with uniform OAM around the center of the packet is extremely low [34], which is certainly true for neutrons in a crystal interferometer, and even for X-rays it is difficult to use a division amplitude interferometer (like the used neutron interferometer) to create a vortex wave front with SPP [35].…”