Classically speaking a particle orbiting around its propagation axis, in the absence of a restoring force seems unphysical, however standard quantum mechanics allows for free massive particles to posses discrete, quantized units of orbital angular momentum (OAM). To date OAM has been observed in free photons [1] and free electrons [2]. In neutrons phase vortices which exhibit extrinsic OAM with respect to the optical axis have been recorded using aluminium spiral phase plates [3]. Furthermore, magnetic methods, using quadrupoles and linear magnetic gradients, have been developed to generate neutron states which exhibit coupling between spin and orbital angular momentum degrees of freedom [4,5]. In the latter case phase vortex lattices can be produced [5], which may prove useful in probing periodic chiral structures. In general neutron OAM would find applications in neutron scattering [6] quantum information and contextuality [7]. Detection of neutron OAM has always relied on using position sensitive detectors to resolve the phase vortices. This presents an obvious problem in the case of intrinsic OAM were the size of the phase vortex is similar to the coherence length of the neutrons. The effectiveness of the aforementioned methods at generating intrinsic OAM is also directly proportional to the neutron coherence length. For this reason there is need for OAM generators and detectors which work well for small neutron coherence lengths or are independent of coherence length. To this end new methods are being explored and developed. Suggested methods for inducing OAM states in neutrons include magnetic dipole-electric field interactions [8], the neutron-nucleus weak interaction [9] and phase gratings. The former two methods rely on the fact that the Schwinger process and the neutron-nucleus weak interaction preserve total angular momentum [10,11]. However, since both interactions do not preserve spin angular momentum it follows that orbital angular momentum must be generated. While this OAM can be parallel to the propagation direction (longitudinal), it can also in theory by transverse [8]. Such transverse OAM states have not yet been observed in any kind of free particles. Neutron OAM detection schemes could exploit the rotational doppler shift [12] and the Fizeau effect [13], in which the energy of a neutron is shifted by lΩ, where l is the OAM quantum number and Ω is the rotational frequency of the system. Due to this energy shift the neutron optical density of a rotating medium will appear different to a neutron carrying OAM. This effect may be amplified if the medium exhibits a resonance close to the incident neutron energy. Furthermore, neutron optical dove prisms [14] using supermirror triplets can be employed. These devices act as mode inverters and rotate the phase vortex by an angle proportional to the OAM quantum number. This OAM dependent phase opens up a new avenue to measure neutron OAM.

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[2] B.J. McMorran et al., Science 331, 192-195 (2011).

[3] C.W. Clark et al., Nature 525, 504-506 (2015).

[4] J. Nsofini el al., Phys. Rev. A 94, 013605 (2016).

[5] D. Sarenac et al., PNAS 116, 20328-20332 (2019).

[6] A.V. Afanasev et al., Phys. Rev. C 100, 051601 (2019).

[7] J. Shen et al., Nat. Commun. 11, 930 (2020).

[8] N. Geerits and S. Sponar ArXiv:2007.03345 (currently under review).

[9] S. Sponar et al., ArXiv 2012.09048.

[10] S.A. Bruce and J.F. Diaz-Valdes, Eur. J. Phys. 41, 045402 (2020).

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[12] L. Allen et al., Optics Communications 112, 141-144 (1994).

[13] S. Franke-Arnold et al., Science 333, 65-67, (2011).

[14] J. Leach et al., Phys. Rev. Lett. 88, 257901 (2002).

Contact : tellier@ill.fr