Structure Neutron Waves: Generating and Detecting Neutrons with Orbital Angular Momentum

Summary

Structured waves for optics, electrons, and atoms have been becoming increasingly useful in a variety of applications including communication, microscopy, and quantum science. Structured waves in the neutron regime are inherently more difficult to generate due to the lack of a coherent neutron source and the neutron’s small transverse coherence. Despite these challenges, NIST has been at the forefront of developing ways to generate neutron structured waves which may have applications in understanding the bulk properties of quantum materials and in quantum information science.

Description

Light, electrons, and even atoms can be imparted with well-defined orbital angular momentum (OAM) which is often described as a ‘twisting’ or helical wavefront (see Fig. 1) as the particle propagates through space. These structured waves have found applications in optical tweezers, quantum information science, and advanced imaging. OAM states are easily generated for light using commercially available refractive and diffractive optics. However, generating such helical wavefronts in neutrons has been a significant challenge due to limitations in neutron optics and coherence.

Neutron-based experiments play a crucial role in material science and fundamental physics due to the unique properties of neutrons: they have no electrical charge, interact with atomic nuclei, and possess an intrinsic spin. The successful generation of neutron helical waves provides a new degree of control over neutron wavefunctions, while the development of OAM-based neutron interferometry enables the recovery of critical phase information in neutron scattering. Together, these advancements pave the way for improved material analysis, more precise quantum experiments, and novel applications in spintronics and structured wave physics.

The first demonstration of neutrons with OAM was done at NIST using a spiral wave plate which imparted an azimuthal phase along a neutron wave. Our latest work involves using fork dislocation gratings to generate OAM. Fork dislocations have been used in optics and can be achieved for neutrons using electron beam lithography on a silicon wafer to produce the unique patterned gratings only hundreds of nanometers high (see Fig. 2). Instead of using a single fork dislocation, we microfabricated arrays containing over 6 million fork dislocation elements. This multiplication of the outgoing OAM states allows for greater intensity and usability. Figure 3 shows  how a beam of OAM = 3 was generated and the distinctive diffraction pattern observed at the detector.

Figure 1:  An example of light propagating with different values of OAM ( m = +/-2, m =+/-1).  The middle set of images are for light without  OAM  (left) The wavefront ‘twists’ as the photon propagates in the direction of the arrow. (middle) The phase profile of the wavefront. (right) The intensity of light measured by a camera.  OAM states generate a distinctive doughnut pattern.

Credit: Taken from Wikimedia under its Creative Commons license

Figure 2: SEM images characterizing the array of fork dislocation phase gratings used to generate the neutron helical wavefronts. The arrays covered an area of 0.5 cm by 0.5 cm and consisted of 6,250,000 individual fork dislocation phase gratings with a period of 120 nm.

Credit: Taken from Sarenac et al., Sci. Adv. 8, eadd2002(2022)

Figure 3: (a) Here two arrays of fork dislocation gratings generate OAM states on a neutron beam, which propagate at an angle with respect to the incident beam, causing interference at the detector (b) Top-down view of the grating setup showing that the array of reference beams needs to spatially overlap the array of object beams to interfere with the camera.

Credit: Taken from Sarenac et al., Nat Commun 15, 10785 (2024)

Implications for Neutron Science

This breakthrough significantly enhances the power of neutron scattering techniques by providing a means to access phase information that was previously unavailable. The potential applications include:

• Improved Material Characterization – The technique can be applied to study the internal structures of complex materials with greater accuracy. Neutron OAM states can probe magnetic skyrmion lattices, which are promising candidates for next-generation data storage.
• Quantum Information Studies – Neutron OAM states allow for an additional degree-of-freedom and offers a unique entanglement pathway thereby enhancing neutron capabilities in quantum information science.
• Alterative Structured Waves – The ability to manipulate and measure neutron wavefronts opens new pathways for developing “self-accelerating” Airy beams and “nondiffractive” Bessel beams; both which have a “self-healing” property as they propagate.

The continued integration of structured neutron waves into neutron science has the potential to revolutionize how researchers explore and manipulate the quantum properties of matter, making neutron-based techniques more powerful and versatile than ever before.