Neutrons do not normally have orbital angular momentum. But the demonstration that a beam of neutrons can acquire this property, 23 years after it was shown in photons, offers the promise of improved imaging technologies. See Letter p.504
Neutrons were discovered in 1932 by the physicist James Chadwick, and the particles continue to amaze scientists to this day. It was initially thought that neutrons were elementary particles — that is, that they were not composed of other particles. But we now know that, just like protons, neutrons are comprised of three elementary particles called quarks. Quarks have an intrinsic property known as spin angular momentum (or spin), and they endow the neutron with a spin that has a value of ½ħ (where ħ is the reduced Planck constant). On page 504 of this issue, Clark et al.1 show that a free neutron can have a different kind of angular momentum: orbital angular momentum (OAM).
OAM is a broad concept in modern physics, but is usually associated with the motion of electrons around the atomic nucleus in atoms and molecules. In contrast to spin, OAM is not an intrinsic property of the electron: it can take any value of an integer L multiplied by ħ, whereas the electron's spin has a fixed value of ½ħ. Electron spin and OAM are analogous to Earth's rotation on its axis and its orbit around the Sun, respectively.
But OAM has also arisen in a different context: in the early 1990s, it was theoretically2 and experimentally3 shown that any helically phased light beam can possess OAM. It has since been established that this is true even for a single photon4. This is therefore another source of angular momentum for the particle, in addition to its spin (which is associated with the circular polarization of light). It is a crucial property of photons that has found applications in the field of photonics, such as the coding of quantum4 and classical5 information in individual photons, quantum-entanglement protocols6 and the manipulation of small particles by optical forces7.
In 2010, electron beams with OAM were also generated, confirming that this property is not limited to light beams8. Many advances in the production and use of OAM-carrying electron beams have since been reported (see ref. 9 for a review). The fact that photons are not the only particles that can have OAM has opened up possibilities for fundamental studies of electromagnetic interactions and for applications such as improved electron microscopes.
Clark and colleagues' work adds neutrons to the list of particles that can have OAM. The authors generate OAM-carrying neutrons by guiding a beam of the particles through a device known as a spiral phase plate (Fig. 1). The thickness of this device varies uniformly as a function of the plate's azimuthal angle, φ (the angle measured around the circumference of the plate). The wavefunction of a neutron passing through this device acquires a phase shift that is proportional to the plate's local thickness. For appropriate values of the variation of thickness with φ, the wavefunction acquires an azimuthal phase distribution given by eiLφ, where L is any positive or negative integer and i is the 'imaginary unit' (the square root of −1).
The authors fabricated several plates whose thickness distributions corresponded to various values of L, and thus generated neutron beams carrying OAM of different Lħ values. Like its spin, a neutron's OAM is a quantum-mechanical attribute. It occurs as a consequence of the helical structure of the particle's 'twisted' wavefunction when it emerges from the plate. To verify that the neutron beam had acquired OAM as it passed through the plate, Clark et al. used a technique known as neutron interferometry. In this approach, the neutron wavefunction was split into two paths and a spiral phase plate was placed in one of them. The two paths were subsequently combined coherently to form an output beam whose interference pattern showed the azimuthal phase distribution that the wavefunction had acquired.
Although Clark and colleagues' results are impressive, they represent only the first step in an emerging field of research. For example, in the present experiment, the neutron beam falling on the spiral phase plate is a statistical mixture of several OAM quantum states. Before applications can be developed, neutrons must be generated that have quantum states with definitive OAM values (eigenstates). In addition, holographic methods have been developed for creating optical10 and electron12 OAM states, and these are more precise and versatile than the use of spiral phase plates. It will thus be interesting to explore the use of holographic techniques for neutrons too. The potential use of neutron OAM states for quantum-information studies is another exciting prospect.
Finally, Clark and colleagues' study opens up a further avenue for future work: the use of neutron beams with OAM for imaging. Because neutrons are penetrating particles, they could offer practical advantages compared with optical and electron microscopy in deep-imaging studies of materials. One might therefore conclude that OAM-carrying neutron beams may boldly go where no quantum particle has gone before.Footnote 1
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