Journal Article

Controlling neutron orbital angular momentum

Cory, David G., Barankov, Roman, Pushin, Dmitry A., Huber, Michael G., Clark, Charles W., Arif, Muhammad

2015

Overview

Interferometry reveals quantized changes in the angular momentum of neutrons that have been ‘twisted’ by passage through a spiral staircase structure. Orbital angular momentum control in neutron optics Orbital angular momentum is a quantized degree of freedom exploited in many applications. The photon orbital angular momentum has been used in fundamental tests of quantum mechanics and imaging and the electron orbital angular momentum has proven useful for determining the chirality of crystals. But the phenomenon had not previously been demonstrated in neutrons. Here Dmitry Pushin and colleagues show how to control orbital angular momentum states in a neutron beam through the use of macroscopic spiral phase plates. After applying this 'twist' to an input neutron beam, the quantized orbital angular momentum of neutrons is characterized by neutron interferometry. In contrast to photons and electrons, neutrons are massive particles, hence this result could open important new perspectives for testing quantum mechanics with massive observables. In addition, the neutron orbital angular momentum could enable new approaches for neutron scattering techniques. The quantized orbital angular momentum (OAM) of photons 1 offers an additional degree of freedom and topological protection from noise. Photonic OAM states have therefore been exploited in various applications 2 , 3 ranging from studies of quantum entanglement and quantum information science 4 , 5 , 6 , 7 to imaging 8 , 9 , 10 , 11 , 12 . The OAM states of electron beams 13 , 14 , 15 have been shown to be similarly useful, for example in rotating nanoparticles and determining the chirality of crystals 16 , 17 , 18 , 19 . However, although neutrons—as massive, penetrating and neutral particles—are important in materials characterization, quantum information and studies of the foundations of quantum mechanics, OAM control of neutrons has yet to be achieved. Here, we demonstrate OAM control of neutrons using macroscopic spiral phase plates that apply a ‘twist’ to an input neutron beam. The twisted neutron beams are analysed with neutron interferometry. Our techniques, applied to spatially incoherent beams, demonstrate both the addition of quantum angular momenta along the direction of propagation, effected by multiple spiral phase plates, and the conservation of topological charge with respect to uniform phase fluctuations. Neutron-based studies of quantum information science 20 , 21 , the foundations of quantum mechanics 22 , 23 , and scattering and imaging 24 of magnetic, superconducting and chiral materials have until now been limited to three degrees of freedom: spin, path and energy. The optimization of OAM control, leading to well defined values of OAM, would provide an additional quantized degree of freedom for such studies.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/766/483/3924 / 639/766/483/481 / Crystals / Humanities and Social Sciences / Interferometry / letter / multidisciplinary / Neutrons / Particle physics / Science