Controlling neutron orbital angular momentum

Water-Wave Vortices and Skyrmions

Topological wave structures-phase vortices, skyrmions, merons, etc.-are attracting enormous attention in a variety of quantum and classical wave fields. Surprisingly, these structures have never been properly explored in the most obvious example of classical waves: water-surface (gravity-capillary) waves. Here, we fill this gap and describe (i) water-wave vortices of different orders carrying quantized angular momentum with orbital and spin contributions, (ii) skyrmion lattices formed by the instantaneous displacements of the water-surface particles in wave interference, and (iii) meron (half-skyrmion) lattices formed by the spin-density vectors, as well as (iv) spatiotemporal water-wave vortices and skyrmions. We show that all these topological entities can be readily generated in linear water-wave interference experiments. Our findings can find applications in microfluidics and show that water waves can be employed as an attainable playground for emulating universal topological wave phenomena.

Spin-orbital angular momentum degeneracy breaking in nanoplasmonic metachain

The spin and orbital angular momentum (namely SAM and OAM) mode division provides a promising solution to surmount exhausted available degrees of freedom in conventional optical communications. Nevertheless, SAM and OAM are often subjected to the degeneracy of total angular momentum (AM) because they both have integer variables of quantum eigenstates, which inevitably brings about the shortcomings specific to limited signal channels and multiplexing cross talk. Herein, we present a nanoplasmonic metachain that can discriminatively couple any input SAM and OAM components to an extrinsic orbital AM, corresponding to the chirality and topological charge of incident light. Importantly, the unambiguous measurement has a prominent advantage of detecting the arbitrary AM component rather than the total AM. The miniature metadevice offers the possibility of harnessing AM division on chip or in fiber and holds great promise to delve the spin-orbit interactions for topological photonics and quantum cryptography.

Development of Neutron Interferometer Using Multilayer Mirrors and Measurements of Neutron-Nuclear Scattering Length with Pulsed Neutron Source

This study entailed the successful deployment of a novel neutron interferometer that utilizes multilayer mirrors. The apparatus facilitates a precise evaluation of the wavelength dependence of interference fringes utilizing a pulsed neutron source. Our interferometer achieved an impressive precision of 0.02 rad within a 20-min recording time. Compared to systems using silicon crystals, the measurement sensitivity was maintained even when using a simplified disturbance suppressor. By segregating beam paths entirely, we achieved successful measurements of neutron-nuclear scattering lengths across various samples. The values measured for Si, Al, and Ti were in agreement with those found in the literature, while V showed a disparity of 45%. This discrepancy may be attributable to impurities encountered in previous investigations. The accuracy of measurements can be enhanced further by mitigating systematic uncertainties that are associated with neutron wavelength, sample impurity, and thickness. This novel neutron interferometer enables us to measure fundamental parameters, such as the neutron-nuclear scattering length of materials, with a precision that surpasses that of conventional interferometers.

Generation of femtosecond optical vortices with multiple separate phase singularities from a Kerr-lens mode-locked Yb:KGW oscillator

Femtosecond optical vortices with a phase singular point have diverse applications such as microscopic particles manipulation, special-structure micro-processing and quantum information. Raising the number of singularity points can provide additional dimensions of control. Here we report for what we believe is the first time the generation of femtosecond optical vortices with multiple (two and five) singularities directly from a laser oscillator. The average powers and pulse durations of the resulting vortex pulses are several hundred milliwatts and less than 300 fs, respectively. This work represents an innovate way for obtaining femtosecond multi-vortices, opening the way to the further studies of optical vortex crystals and their applications.

Topologically crafted spatiotemporal vortices in acoustics

Vortices in fluids and gases have piqued the human interest for centuries. Development of classical-wave physics and quantum mechanics highlighted wave vortices characterized by phase singularities and topological charges. In particular, vortex beams have found numerous applications in modern optics and other areas. Recently, optical spatiotemporal vortex states exhibiting the phase singularity both in space and time have been described. Here, we report the topologically robust generation of acoustic spatiotemporal vortex pulses. We utilize an acoustic meta-grating with broken mirror symmetry which exhibits a topological phase transition with a pair of phase singularities with opposite topological charges emerging in the momentum-frequency domain. We show that these vortices are topologically robust against structural perturbations of the meta-grating and can be employed for the generation of spatiotemporal vortex pulses. Our work paves the way for studies and applications of spatiotemporal structured waves in acoustics and other wave systems.

Twisted moiré photonic crystal enabled optical vortex generation through bound states in the continuum

The twisted stacking of two layered crystals has led to the emerging moiré physics as well as intriguing chiral phenomena such as chiral phonon and photon generation. In this work, we identified and theoretically formulated a non-trivial twist-enabled coupling mechanism in twisted bilayer photonic crystal (TBPC), which connects the bound state in the continuum (BIC) mode to the free space through the twist-enabled channel. Moreover, the radiation from TBPC hosts an optical vortex in the far field with both odd and even topological orders. We quantitatively analyzed the twist-enabled coupling between the BIC mode and other non-local modes in the photonic crystals, giving rise to radiation carrying orbital angular momentum. The optical vortex generation is robust against geometric disturbance, making TBPC a promising platform for well-defined vortex generation. As a result, TBPCs not only provide a new approach to manipulating the angular momentum of photons, but may also enable novel applications in integrated optical information processing and optical tweezers. Our work broadens the field of moiré photonics and paves the way toward the novel application of moiré physics.

Optical vortices shape optical tornados

We demonstrate that by seeding an accelerating ring-Airy beam with a finite number of off-axis optical vortices, it transforms into a tornado wave (ToW) upon propagation. Using numerical simulations, we show that both the spiraling high-intensity lobes and the optical vortices exhibit angular acceleration and follow interwinding braid-like trajectories. Likewise, we study the effect of the number, position, and topological charge of the vortices on the propagation dynamics and reveal the connection between optical vortices and optical tornados.

Phase vortex lattices in neutron interferometry

Neutron Orbital Angular Momentum (OAM) is an additional quantum mechanical degree of freedom, useful in quantum information, and may provide more complete information on the neutron scattering amplitude of nuclei. Various methods for producing OAM in neutrons have been discussed. In this work we generalize magnetic methods which employ coherent averaging and apply this to neutron interferometry. Two aluminium prisms are inserted into a nested loop interferometer to generate a phase vortex lattice with significant extrinsic OAM, 〈Lz〉 ≈ 0.35, on a length scale of ≈ 220 μm, transverse to the propagation direction. Our generalized method exploits the strong nuclear interaction, enabling a tighter lattice. Combined with recent advances in neutron compound optics and split crystal interferometry our method may be applied to generate intrinsic neutron OAM states. Finally, we assert that, in its current state, our setup is directly applicable to anisotropic ultra small angle neutron scattering.

Quantum Interference between Fundamentally Different Processes Is Enabled by Shaped Input Wavefunctions

This work presents a general framework for quantum interference between processes that can involve different fundamental particles or quasi-particles. This framework shows that shaping input wavefunctions is a versatile and powerful tool for producing and controlling quantum interference between distinguishable pathways, beyond previously explored quantum interference between indistinguishable pathways. Two examples of quantum interference enabled by shaping in interactions between free electrons, bound electrons, and photons are presented: i) the vanishing of the zero-loss peak by destructive quantum interference when a shaped electron wavepacket couples to light, under conditions where the electron's zero-loss peak otherwise dominates; ii) quantum interference between free electron and atomic (bound electron) spontaneous emission processes, which can be significant even when the free electron and atom are far apart, breaking the common notion that a free electron and an atom must be close by to significantly affect each other's processes. Conclusions show that emerging quantum wave-shaping techniques unlock the door to greater versatility in light-matter interactions and other quantum processes in general.

Magnonic Einstein-de Haas Effect: Ultrafast Rotation of Magnonic Microspheres

Magnons, collective spin excitations in magnetic crystals, have attracted much interest due to their ability to couple strongly to microwaves and other quantum systems. In compact magnetic crystals, we show that there are magnonic modes that can support orbital angular momentum and that these modes can be driven by linearly polarized microwave fields. Because of conservation of angular momentum, exciting such magnon modes induces a mechanical torque on the crystal. We study a levitated magnetic crystal, a yttrium iron garnet (YIG) microsphere, where such orbital angular momentum magnon modes are driven by microwaves held in a microwave high-Q microwave cavity. We find that the YIG sphere experiences a mechanical torque and can be spun up to ultralarge angular speeds exceeding 10 GHz.

Experimental realization of neutron helical waves

Methods of preparation and analysis of structured waves of light, electrons, and atoms have been advancing rapidly. Despite the proven power of neutrons for material characterization and studies of fundamental physics, neutron science has not been able to fully integrate these techniques because of small transverse coherence lengths, the relatively poor resolution of spatial detectors, and low fluence rates. Here, we demonstrate methods that are practical with the existing technologies and show the experimental achievement of neutron helical wavefronts that carry well-defined orbital angular momentum values. We discuss possible applications and extensions to spin-orbit correlations and material characterization techniques.

Twisted Magnon Frequency Comb and Penrose Superradiance

Quantization effects of the nonlinear magnon-vortex interaction in ferromagnetic nanodisks are studied. We show that the circular geometry twists the spin-wave fields with spiral phase dislocations carrying quantized orbital angular momentum (OAM). Meanwhile, the confluence and splitting scattering of twisted magnons off the gyrating vortex core (VC) generates a frequency comb consisting of discrete and equally spaced spectral lines, dubbed as twisted magnon frequency comb (TMFC). It is found that the mode spacing of the TMFC is equal to the gyration frequency of the VC and the OAM quantum numbers between adjacent spectral lines differ by one. By applying a magnetic field perpendicular to the plane of a thick nanodisk, we observe a magnonic Penrose superradiance inside the cone vortex state, which mimics the amplification of particles scattered from a rotating black hole. It is demonstrated that the higher-order modes of TMFC are significantly amplified while the lower-order ones are trapped within the VC gyrating orbit which manifests as the ergoregion. These results suggest a promising way to generate twisted magnons with large OAM and to drastically improve the flatness of the magnon comb.

Method for the definitive detection of orbital angular momentum states in neutrons by spin-polarized 3He

A standard method to detect thermal neutrons is the nuclear interaction 3He(n,p)3H. The spin dependence of this interaction is also the basis of a neutron spin-polarization filter using nuclear polarized 3He. We consider the corresponding interaction for neutrons placed in an intrinsic orbital angular momentum (OAM) state. We derive the relative polarization-dependent absorption cross sections for neutrons in an L = 1 OAM state. The absorption of those neutrons results in compound states J π = 0 - , 1 - , and 2 - . Varying the three available polarizations tests that an OAM neutron has been absorbed and probes which decay states are physically possible. We describe the energetically likely excited states of 4He after absorption, taking account of the odd parity of the compound state. This provides a definitive method for detecting neutron OAM states and suggests that intrinsic OAM states offer the possibility to observe new physics, including anomalous cross sections and new channels of radioactive decay.

Dynamical Control of Nuclear Isomer Depletion via Electron Vortex Beams

Some nuclear isomers are known to store a large amount of energy over long periods of time, with a very high energy-to-mass ratio. Here, we describe a protocol to achieve the external control of the isomeric nuclear decay by using electron vortex beams whose wave function has been especially designed and reshaped on demand. Recombination of these electrons into the isomer's atomic shell can lead to the controlled release of the stored nuclear energy. On the example of ^{93m}Mo, we show theoretically that the use of tailored electron vortex beams increases the depletion by 4 orders of magnitude compared to the spontaneous nuclear decay of the isomer. Furthermore, specific orbitals can sustain an enhancement of the recombination cross section for vortex electron beams by as much as 6 orders of magnitude, providing a handle for manipulating the capture mechanism. These findings open new prospects for controlling the interplay between atomic and nuclear degrees of freedom, with potential energy-related and high-energy radiation source applications.

Searches for Exotic Interactions Using Neutrons

Slow neutrons possess several advantageous properties which make them useful probes for a variety of exotic interactions, including some that can form at least some components of the dark matter of interest for this issue of Symmetry. We discuss the relevant neutron properties, describe some of the recent work that has been done along these lines using neutron experiments mainly with cold and ultra-cold neutrons, and outline some interesting and exciting opportunities which can be pursued using resonant epithermal neutron interactions in heavy nuclei.

Vortex beams of atoms and molecules

[Figure: see text].

Rotational Doppler shift from a rotating rod

This Letter reports on a rotational Doppler effect obtained from a rotating rod illuminated by a fundamental Gaussian laser beam. More specifically, we decompose the transmitted light behind the rotating rod into Laguerre-Gaussian modes and investigate the associated frequency shifts. The main contributing modes correspond to modes having the same rotational symmetry as the rotating object. Furthermore, their shifts equal the topological charge of the beam times the rotational frequency of the object. Potential applications in pattern recognition and rotation identification are then considered.

Experimental demonstration and investigation of vortex circular Pearcey beams in a dynamically shaped fashion

Optical vortex, typically characterized by a helical phase front, results in a possession of orbital angular momentum. In recent years, teleportation of the vortex mode using novel beams with peculiar features has gained great interest. Here, we experimentally demonstrate the propagation dynamics for a new class of the auto-focusing vortex circular Pearcey beam (VCPB), which is theoretically described by delivering the coaxial or off-axial spiral phases into the circular Pearcey beam (CPB), forming the crescent or bottle-like focal structure with self-rotation. Notably, such a hybrid beam with various types is experimentally obtained through a digital micromirror device (DMD) with the binary amplitude holography, and this DMD-based modulation scheme combined with controllable vortex modes enables dynamic switching among the VCPBs. We also measure the topological phase by interferometry and we explain the beam property on the basis of Poynting vector, showing a good agreement with the simulations. Further, the number, location and mode of embedded vortices could offer multiple dimensions of flexibility for target beam modulation, thus the experimentally controllable VCPBs will bring potential to high-speed optical communications and particle manipulations that require dynamic shaping.

Fractional Angular Momenta, Gouy and Berry Phases in Relativistic Bateman-Hillion-Gaussian Beams of Electrons

A new Bateman-Hillion solution to the Dirac equation for a relativistic Gaussian electron beam taking explicit account of the four-position of the beam waist is presented. This solution has a pure Gaussian form in the paraxial limit but beyond it contains higher order Laguerre-Gaussian components attributable to the tighter focusing. One implication of the mixed mode nature of strongly diffracting beams is that the expectation values for spin and orbital angular momenta are fractional and are interrelated to each other by intrinsic spin-orbit coupling. Our results for these properties align with earlier work on Bessel beams [Bliokh et al., Phys. Rev. Lett. 107, 174802 (2011)PRLTAO0031-900710.1103/PhysRevLett.107.174802] and show that fractional angular momenta can be expressed by means of a Berry phase. The most significant difference arises, though, due to the fact that Laguerre-Gaussian beams naturally contain Gouy phase, while Bessel beams do not. We show that Gouy phase is also related to Berry phase and that Gouy phase fronts that are flat in the paraxial limit become curved beyond it.

Control of quantum electrodynamical processes by shaping electron wavepackets

Fundamental quantum electrodynamical (QED) processes, such as spontaneous emission and electron-photon scattering, encompass phenomena that underlie much of modern science and technology. Conventionally, calculations in QED and other field theories treat incoming particles as single-momentum states, omitting the possibility that coherent superposition states, i.e., shaped wavepackets, can alter fundamental scattering processes. Here, we show that free electron waveshaping can be used to design interferences between two or more pathways in a QED process, enabling precise control over the rate of that process. As an example, we show that free electron waveshaping modifies both spatial and spectral characteristics of bremsstrahlung emission, leading for instance to enhancements in directionality and monochromaticity. The ability to tailor general QED processes opens up additional avenues of control in phenomena ranging from optical excitation (e.g., plasmon and phonon emission) in electron microscopy to free electron lasing in the quantum regime.

Exact design of complex amplitude holograms for producing arbitrary scalar fields

Typical methods to holographically encode arbitrary wavefronts assume the hologram medium only applies either phase shifts or amplitude attenuation to the wavefront. In many cases, phase cannot be introduced to the wavefront without also affecting the amplitude. Here we show how to encode an arbitrary wavefront into an off-axis transmission hologram that returns the exact desired arbitrary wavefunction in a diffracted beam for phase-only, amplitude-only, or mixed phase and amplitude holograms with any periodic groove profile. We apply this to design thin holograms for electrons in a TEM, but our results are generally applicable to light and X-ray optics. We employ a phase reconstruction from a series of focal plane images to qualitatively show the accuracy of this method to impart the expected amplitude and phase to a specific diffraction order.

Twisted Magnon as a Magnetic Tweezer

Wave fields with spiral phase dislocations carrying orbital angular momentum (OAM) have been realized in many branches of physics, such as for photons, sound waves, electron beams, and neutrons. However, the OAM states of magnons (spin waves)-the building block of modern magnetism-and particularly their implications have yet to be addressed. Here, we theoretically investigate the twisted spin-wave generation and propagation in magnetic nanocylinders. The OAM nature of magnons is uncovered by showing that the spin-wave eigenmode is also the eigenstate of the OAM operator in the confined geometry. Inspired by optical tweezers, we predict an exotic "magnetic tweezer" effect by showing skyrmion gyrations under twisted magnons in the exchange-coupled nanocylinder-nanodisk heterostructure, as a practical demonstration of magnonic OAM transfer to manipulate topological spin defects. Our study paves the way for the emerging magnetic manipulations by harnessing the OAM degree of freedom of magnons.

A Photonic crystal fiber with large effective refractive index separation and low dispersion

A photonic crystal fiber (PCF) structure with a ring-core and 5 well-ordered semiellipse air-holes has been creatively proposed. Through a comparison between the structures with a high refractive index (RI) ring-core and the structure without, it conclude that a PCF with a high RI ring-core can work better. Schott SF57 was elected as the substrate material of ring-core. This paper compares the effects of long-axis and short-axis changes on the PCF and selects the optimal solution. Especially TE0,1 mode's dispersion is maintained between 0 and 3 ps / (nm · km) ranging from 1.45 μm to 1.65 μm. This property can be used to generate a supercontinuum with 200 μm long zero dispersion wavelength (ZDM). In addition, Δneff reaches up to 10-3, which enables the near -degeneracy of the eigenmodes to be almost neglected. The proposed PCF structure will have great application value in the field of optical communications.

Unveiling contextual realities by microscopically entangling a neutron

The development of qualitatively new measurement capabilities is often a prerequisite for critical scientific and technological advances. Here we introduce an unconventional quantum probe, an entangled neutron beam, where individual neutrons can be entangled in spin, trajectory and energy. The spatial separation of trajectories from nanometers to microns and energy differences from peV to neV will enable investigations of microscopic magnetic correlations in systems with strongly entangled phases, such as those believed to emerge in unconventional superconductors. We develop an interferometer to prove entanglement of these distinguishable properties of the neutron beam by observing clear violations of both Clauser-Horne-Shimony-Holt and Mermin contextuality inequalities in the same experimental setup. Our work opens a pathway to a future of entangled neutron scattering in matter.

Exploring correlations in strongly entangled quantum materials is of interest. Here the authors generate a tunable spin-, trajectory-, and energy-entangled neutron beam using a neutron spin-echo interferometer and show violations of Clauser-Horne-Shimony-Holt and Mermin contextuality inequalities with micron-scale trajectory separation.

Realization of all-optical vortex switching in exciton-polariton condensates

Vortices are topological objects representing the circular motion of a fluid. With their additional degree of freedom, the vorticity, they have been widely investigated in many physical systems and different materials for fundamental interest and for applications in data storage and information processing. Vortices have also been observed in non-equilibrium exciton-polariton condensates in planar semiconductor microcavities. There they appear spontaneously or can be created and pinned in space using ring-shaped optical excitation profiles. However, using the vortex state for information processing not only requires creation of a vortex but also efficient control over the vortex after its creation. Here we demonstrate a simple approach to control and switch a localized polariton vortex between opposite states. In our scheme, both the optical control of vorticity and its detection through the orbital angular momentum of the emitted light are implemented in a robust and practical manner.

Generation and detection of spin-orbit coupled neutron beams

Spin-orbit coupling of light has come to the fore in nanooptics and plasmonics, and is a key ingredient of topological photonics and chiral quantum optics. We demonstrate a basic tool for incorporating analogous effects into neutron optics: the generation and detection of neutron beams with coupled spin and orbital angular momentum. The 3He neutron spin filters are used in conjunction with specifically oriented triangular coils to prepare neutron beams with lattices of spin-orbit correlations, as demonstrated by their spin-dependent intensity profiles. These correlations can be tailored to particular applications, such as neutron studies of topological materials.

Twisted magnon beams carrying orbital angular momentum

Low-energy eigenmode excitations of ferromagnets are spin waves or magnons that can be triggered and guided in magnonic circuits without Ohmic losses and hence are attractive for communicating and processing information. Here we present new types of spin waves that carry a definite and electrically controllable orbital angular momentum (OAM) constituting twisted magnon beams. We show how twisted beams emerge in magnonic waveguides and how to topologically quantify and steer them. A key finding is that the topological charge associated with OAM of a particular beam is tunable externally and protected against magnetic damping. Coupling to an applied electric field via the Aharanov-Casher effect allows for varying the topological charge. This renders possible OAM-based robust, low-energy consuming multiplex magnonic computing, analogously to using photonic OAM in optical communications, and high OAM-based entanglement studies, but here at shorter wavelengths, lower energy consumption, and ready integration in magnonic circuits.

Laguerre-Gaussian mode sorter

Exploiting a particular wave property for a particular application necessitates components capable of discriminating in the basis of that property. While spectral or polarisation decomposition can be straightforward, spatial decomposition is inherently more difficult and few options exist regardless of wave type. Fourier decomposition by a lens is a rare simple example of a spatial decomposition of great practical importance and practical simplicity; a two-dimensional decomposition of a beam into its linear momentum components. Yet this is often not the most appropriate spatial basis. Previously, no device existed capable of a two-dimensional decomposition into orbital angular momentum components, or indeed any discrete basis, despite it being a fundamental property in many wave phenomena. We demonstrate an optical device capable of decomposing a beam into a Cartesian grid of identical Gaussian spots each containing a single Laguerre-Gaussian component, using just a spatial light modulator and mirror.

Compton Scattering of γ-Ray Vortex with Laguerre Gaussian Wave Function

In this work, we report calculation for Compton scattering of a γ-ray vortex with a wave function of Laguerre Gaussian on an electron in the framework of the relativistic quantum mechanics. We consider the coincidence measurement of the scattered photon and the scattered electron from each Compton scattering. The momentum of the scattered photon distributes outside of the reaction plane determined by the incident photon and the scattered electron, and the energy of the scattered photon also distributes, when the scattered angle of the electron is simultaneously measured. These distributions depend on the angular momentum and the node number of the Laguerre Gaussian function of the incident photon. Thus, the coincident measurement for Compton scattering is useful to identify the nature of the vortex photon wave function.

Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities

Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin-orbital interactions, Bose-Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications.

Ptychographic amplitude and phase reconstruction of bichromatic vortex beams

We experimentally demonstrate that ptychographic coherent diffractive imaging can be used to simultaneously characterize the amplitude and phase of bichromatic orbital angular momenta-shaped vortex beams, which consist of a fundamental field, together with its copropagating second-harmonic field. In contrast to most other orbital angular momentum characterization methods, this approach solves for the complex field of a hyperspectral beam. This technique can also be used to characterize other phase-structured illumination beams, and, in the future, will be able to be extended to other complex fields in the extreme ultraviolet or X-ray spectral regions, as well as to matter waves.

Generation of a Lattice of Spin-Orbit Beams via Coherent Averaging

We describe a highly robust method, applicable to both electromagnetic and matter-wave beams, that can produce a beam consisting of a lattice of orbital angular momentum (OAM) states coupled to a two-level system. We also define efficient protocols for controlling and manipulating the lattice characteristics. These protocols are applied in an experimental realization of a lattice of optical spin-orbit beams. The novel passive devices we demonstrate here are also a natural alternative to existing methods for producing single-axis OAM and spin-orbit beams. Our techniques provide new tools for investigations of chiral and topological materials with light and particle beams.

Interplay between topological phase and self-acceleration in a vortex symmetric Airy beam

Photons in an optical vortex usually carry orbital angular momentum, which boosts the application of the micro-rotation of absorbing particles and quantum information encoding. Such photons propagate along a straight line in free space or follow a curved trace once guided by an optical fiber. Teleportation of an optical vortex using a beam with non-diffraction and self-healing is quite challenging. We demonstrate the manipulation of the propagation trace of an optical vortex with a symmetric Airy beam (SAB) and found that the SAB experiences self-rotation with the implementation of a topological phase structure of coaxial vortex. Slight misalignment of the vortex and the SAB enables the guiding of the vortex into one of the self-accelerating channels. Multiple off-axis vortices embedded in SAB are also demonstrated to follow the trajectory of the major lobe for the SAB beam. The Poynting vector for the beams proves the direction of the energy flow corresponding to the intensity distribution. Hence, we anticipate that the proposed vortex symmetric Airy beam (VSAB) will provide new possibilities for optical manipulation and optical communication.

Three Phase-Grating Moiré Neutron Interferometer for Large Interferometer Area Applications

We demonstrate a three phase-grating moiré neutron interferometer in a highly intense neutron beam as a robust candidate for large area interferometry applications and for the characterization of materials. This novel far-field moiré technique allows for broad wavelength acceptance and relaxed requirements related to fabrication and alignment, thus circumventing the main obstacles associated with perfect crystal neutron interferometry. We observed interference fringes with an interferometer length of 4 m and examined the effects of an aluminum 6061 alloy sample on the coherence of the system. Experiments to measure the autocorrelation length of samples and the universal gravitational constant are proposed and discussed.

Intrinsic Orbital Angular Momentum States of Neutrons

It has been shown that single-particle wave functions, of both photons and electrons, can be created with a phase vortex, i.e., an intrinsic orbital angular momentum (OAM). A recent experiment has claimed similar success using neutrons [C. W. Clark et al., Nature, 525, 504 (2015)NATUAS0028-083610.1038/nature15265]. We show that their results are insufficient to unambiguously demonstrate OAM, and they can be fully explained as phase contrast interference patterns. Furthermore, given the small transverse coherence length of the neutrons in the original experiment, the probability that any neutron was placed in an OAM state is vanishingly small. We highlight the importance of the relative size of the coherence length, which presents a unique challenge for neutron experiments compared to electron or photon work, and we suggest improvements for the creation of neutron OAM states.

Increased interference fringe visibility from the post-fabrication heat treatment of a perfect crystal silicon neutron interferometer

We find that annealing a previously chemically etched interferometer at 800 °C dramatically increased the interference fringe visibility from 23% to 90%. The Bragg plane misalignments were also measured before and after annealing using neutron rocking curves, showing that Bragg plane alignment was improved across the interferometer after annealing. This suggests that current interferometers with low fringe visibility may be salvageable and that annealing may become an important step in the fabrication process of future neutron interferometers, leading to less need for chemical etching and larger more exotic neutron interferometers.

Wavelength-tunable Hermite-Gaussian modes and an orbital-angular-momentum-tunable vortex beam in a dual-off-axis pumped Yb:CALGO laser

A dual-off-axis pumping scheme is presented to generate wavelength-tunable high-order Hermite-Gaussian (HG) modes in Yb:CaGdAlO₄ lasers. The mode and wavelength can be actively controlled by the off-axis displacements and pump power. The purities of the output HG modes are quantified by intensity distributions and the measured M² values. The highest order reaches m=15 for stable HG_m,0 mode, and wavelength-tunable width is about 10 nm. Moreover, through externally converting the HG_m,0 modes, the vortex beams carrying orbital angular momentum (OAM) with a large OAM-tunable range from ±1ℏ to ±15ℏ are produced. This work is effective for largely scaling the spectral and OAM tunable ranges of optical vortex beams.

Methods for preparation and detection of neutron spin-orbit states

The generation and control of neutron orbital angular momentum (OAM) states and spin correlated OAM (spin-orbit) states provides a powerful probe of materials with unique penetrating abilities and magnetic sensitivity. We describe techniques to prepare and characterize neutron spin-orbit states, and provide a quantitative comparison to known procedures. The proposed detection method directly measures the correlations of spin state and transverse momentum, and overcomes the major challenges associated with neutrons, which are low flux and small spatial coherence length. Our preparation techniques, utilizing special geometries of magnetic fields, are based on coherent averaging and spatial control methods borrowed from nuclear magnetic resonance. The described procedures may be extended to other probes such as electrons and electromagnetic waves.

Possibility to Probe Negative Values of a Wigner Function in Scattering of a Coherent Superposition of Electronic Wave Packets by Atoms

Within a plane-wave approximation in scattering, an incoming wave packet's Wigner function stays positive everywhere, which obscures such purely quantum phenomena as nonlocality and entanglement. With the advent of the electron microscopes with subnanometer-sized beams, one can enter a genuinely quantum regime where the latter effects become only moderately attenuated. Here we show how to probe negative values of the Wigner function in scattering of a coherent superposition of two Gaussian packets with a nonvanishing impact parameter between them (a Schrödinger's cat state) by atomic targets. For hydrogen in the ground 1s state, a small parameter of the problem, a ratio a/σ_{⊥} of the Bohr radius a to the beam width σ_{⊥}, is no longer vanishing. We predict an azimuthal asymmetry of the scattered electrons, which is found to be up to 10%, and argue that it can be reliably detected. The production of beams with the not-everywhere-positive Wigner functions and the probing of such quantum effects can open new perspectives for noninvasive electron microscopy, quantum tomography, particle physics, and so forth.

Orbital angular momentum mode groups multiplexing transmission over 2.6-km conventional multi-mode fiber

Twisted light carrying orbital angular momentum (OAM) is a special kind of structured light that has a helical phase front, a phase singularity, and a doughnut intensity profile. Beyond widespread developments in manipulation, microscopy, metrology, astronomy, nonlinear and quantum optics, OAM-carrying twisted light has seen emerging application of optical communications in free space and specially designed fibers. Instead of specialty fibers, here we show the direct use of a conventional graded-index multi-mode fiber (MMF) for OAM communications. By exploiting fiber-compatible mode exciting and filtering elements, we excite the first four OAM mode groups in an MMF. We demonstrate 2.6-km MMF transmission using four data-carrying OAM mode groups (OAM_0,1, OAM_+1,1/OAM_-1,1, OAM_+2,1, OAM_+3,1). Moreover, we demonstrate two data-carrying OAM mode groups multiplexing transmission over the 2.6-km MMF with low-level crosstalk free of multiple-input multiple-output digital signal processing (MIMO-DSP). The demonstrations may open up new perspectives to fiber-based OAM communication/non-communication applications using already existing conventional fibers.

Orbital-Angular-Momentum Mode Selection by Rotationally Symmetric Superposition of Chiral States with Application to Electron Vortex Beams

A general orbital-angular-momentum (OAM) mode selection principle is put forward involving the rotationally symmetric superposition of chiral states. This principle is not only capable of explaining the operation of vortex generating elements such as spiral zone plate holograms, but more importantly, it enables the systematic and flexible generation of structured OAM waves in general. This is demonstrated both experimentally and theoretically in the context of electron vortex beams using rotationally symmetric binary amplitude chiral sieve masks.

Nonuniform Currents and Spins of Relativistic Electron Vortices in a Magnetic Field

We present a relativistic description of electron vortex beams in a homogeneous magnetic field. Including spin from the beginning reveals that spin-polarized electron vortex beams have a complicated azimuthal current structure, containing small rings of counterrotating current between rings of stronger corotating current. Contrary to many other problems in relativistic quantum mechanics, there exists a set of vortex beams with exactly zero spin-orbit mixing in the highly relativistic and nonparaxial regime. The well-defined phase structure of these beams is analogous to simpler scalar vortex beams, owing to the protection by the Zeeman effect. For states that do show spin-orbit mixing, the spin polarization across the beam is nonuniform rendering the spin and orbital degrees of freedom inherently inseparable.

Relativistic Electron Vortices

The desire to push recent experiments on electron vortices to higher energies leads to some theoretical difficulties. In particular the simple and very successful picture of phase vortices of vortex charge ℓ associated with ℓℏ units of orbital angular momentum per electron is challenged by the facts that (i) the spin and orbital angular momentum are not separately conserved for a Dirac electron, which suggests that the existence of a spin-orbit coupling will complicate matters, and (ii) that the velocity of a Dirac electron is not simply the gradient of a phase as it is in the Schrödinger theory suggesting that, perhaps, electron vortices might not exist at a fundamental level. We resolve these difficulties by showing that electron vortices do indeed exist in the relativistic theory and show that the charge of such a vortex is simply related to a conserved orbital part of the total angular momentum, closely related to the familiar situation for the orbital angular momentum of a photon.

Normal modes and mode transformation of pure electron vortex beams

Electron vortex beams constitute the first class of matter vortex beams which are currently routinely produced in the laboratory. Here, we briefly review the progress of this nascent field and put forward a natural quantum basis set which we show is suitable for the description of electron vortex beams. The normal modes are truncated Bessel beams (TBBs) defined in the aperture plane or the Fourier transform of the transverse structure of the TBBs (FT-TBBs) in the focal plane of a lens with the said aperture. As these modes are eigenfunctions of the axial orbital angular momentum operator, they can provide a complete description of the two-dimensional transverse distribution of the wave function of any electron vortex beam in such a system, in analogy with the prominent role Laguerre-Gaussian (LG) beams played in the description of optical vortex beams. The characteristics of the normal modes of TBBs and FT-TBBs are described, including the quantized orbital angular momentum (in terms of the winding number l) and the radial index p>0. We present the experimental realization of such beams using computer-generated holograms. The mode analysis can be carried out using astigmatic transformation optics, demonstrating close analogy with the astigmatic mode transformation between LG and Hermite-Gaussian beams.This article is part of the themed issue 'Optical orbital angular momentum'.

Experimental Demonstration of Direct Path State Characterization by Strongly Measuring Weak Values in a Matter-Wave Interferometer

A method was recently proposed and experimentally realized for characterizing a quantum state by directly measuring its complex probability amplitudes in a particular basis using so-called weak values. Recently, Vallone and Dequal [Phys. Rev. Lett. 116, 040502 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.040502] showed theoretically that weak measurements are not a necessary condition to determine the weak value. Here, we report a measurement scheme used in a matter-wave interferometric experiment in which the neutron path system's quantum state was characterized via direct measurements, using both strong and weak interactions. Experimental evidence is given that strong interactions outperform weak ones for tomographic accuracy. Our results are not limited to neutron interferometry, but can be used in a wide range of quantum systems.

Noise refocusing in a five-blade neutron interferometer

We provide a quantum information description of a proposed five-blade neutron interferometer geometry and show that it is robust against low-frequency mechanical vibrations and dephasing due to the dynamical phase. The extent to which the dynamical phase affects the contrast in a neutron interferometer is experimentally shown. In our model, we consider the coherent evolution of a neutron wavepacket in an interferometer crystal blade and simulate the effect of mechanical vibrations and momentum spread of the neutron through the interferometer. The standard three-blade neutron interferometer is shown to be immune to dynamical phase noise but prone to noise from mechanical vibrations, and the decoherence free subspace four-blade neutron interferometer is shown to be immune to mechanical vibration noise but prone to noise from the dynamical phase. Here, we propose a five-blade neutron interferometer and show that it is immune to both low-frequency mechanical vibration noise and dynamical phase noise.

Holography with a neutron interferometer

We use a Mach-Zehnder interferometer to perform neutron holography of a spiral phase plate. The object beam passes through a spiral phase plate, acquiring the phase twist characteristic of orbital angular momentum states. The reference beam passes through a fused silica prism, acquiring a linear phase gradient. The resulting hologram is a fork dislocation image, which could be used to reconstruct neutron beams with various orbital angular momenta. This work paves the way for novel applications of neutron holography, diffraction and imaging.

Transfer of optical orbital angular momentum to a bound electron

Photons can carry angular momentum, not only due to their spin, but also due to their spatial structure. This extra twist has been used, for example, to drive circular motion of microscopic particles in optical tweezers as well as to create vortices in quantum gases. Here we excite an atomic transition with a vortex laser beam and demonstrate the transfer of optical orbital angular momentum to the valence electron of a single trapped ion. We observe strongly modified selection rules showing that an atom can absorb two quanta of angular momentum from a single photon: one from the spin and another from the spatial structure of the beam. Furthermore, we show that parasitic ac-Stark shifts from off-resonant transitions are suppressed in the dark centre of vortex beams. These results show how light's spatial structure can determine the characteristics of light–matter interaction and pave the way for its application and observation in other systems.

The spatial structure of vortex laser beams associates angular momentum to photons, which, in addition to their spin, can be used to tailor light-matter interactions. Here, the authors excite an atomic transition with a vortex laser beam, showing that the transfer of angular momentum modifies selection rules.