twisted light

A door lock with two different keys: one turns easily, the other demands noticeable effort. That is how two layers of hexagonal boron nitride behave when twisted relative to each other. This material consists of boron and nitrogen atoms arranged in a hexagonal lattice, and its layers can sit in different registries, like two identical grids shifted by different angles.

The mirror races along at nearly forty thousand kilometers per second. Light that strikes it bounces back, but changed, like a ball volleyed by a racket in mid-swing. That's exactly the kind of mirror scientists have built inside an ultra-thin glass thread filled with hydrogen. First, a powerful laser pulse is sent down the fiber.

A regular laser beam travels straight, like the light from a flashlight. But light can be twisted into a spiral, like the thread of a corkscrew. Such a beam carries orbital angular momentum: it doesn't just illuminate, it also spins anything it encounters.

A light beam usually travels in a straight line, but you can make it spin around its axis, like water swirling into a vortex as it drains. This 'twisted' light carries more information and is needed for ultra-fast communication and quantum computing.

In a ring accelerator, a beam of electrons circles millions of times. If the electrons are twisted like strands in a rope, their vortex motion should persist. But along the way, random jolts from imperfect magnets can disturb them. Scientists have described how vortex motion changes under such perturbations.

A laser beam is like a swift vortex that briefly captures a passing electron. As the particle moves through the pulse, its spin (the intrinsic rotation, like a tiny top) experiences a weak kick. After leaving the beam, the top no longer points in the same direction; it has turned a little.

An electron is not just a point. It is a probability cloud, smeared out in space. When such a cloud collides with a powerful laser pulse, it can emit a high-energy photon — a gamma quantum. And this photon can be not just ordinary but twisted, like a corkscrew, carrying orbital angular momentum.

A tightly twisted phone cord snaps and scatters sparks. Magnetic fields on the Sun behave the same way: they twist, break, and reconnect, powering flares. For the first time, astronomers have witnessed this process directly. One loop was twisted by 540 degrees — one full turn and then half again. That’s three times more than previously thought possible.

460 light-years away, around the star HD 135344B, a dense disk of gas and dust is swirling. It looks like a giant vinyl record that someone has cracked: a gap yawns in the disk, and across it, like a slender bridge, stretches a dust filament.

Two ultra-thin sheets, each just one atom thick, are placed on top of each other and slightly rotated. Like two sheets of colored tracing paper: while they lie flat, the picture is one thing, but give them a twist and a moiré pattern appears. Here, though, the pattern isn't visible to the eye, but it completely changes how the material handles light.

A flashlight beam glides over uneven glass. The glass is massive galaxy clusters, and the flashlight is ancient light born shortly after the big bang. Passing by such clusters, light doesn't just bend around them like water around a rock. Its path shifts slightly sideways, and the amount of shift depends on how the light is 'twisted'—right or left.

Light, like a shaken rope, oscillates in a specific plane. Astronomers and engineers often rotate images using a three-mirror system, the so-called K-mirror, but this inadvertently rotates the plane of oscillation as well. For precise measurements, that's a problem.

A small ball with an iron core lies on a surface while a magnetic field spins around it. The ball starts rolling like a wheel and usually goes in the direction the field rotates. But sometimes it suddenly turns around and rolls the other way — against the field.

Light usually pushes an atom straight ahead, like wind pushing a sail. But if the beam is twisted into a corkscrew, the picture changes: the atom can fly off to the side, as if caught by a spinning wheel. In the new work, physicists calculated that such a vortex photon can transfer almost all of its rotation to the atom — up to 100 percent.

Think of a stack of paper: when the sheets lie perfectly aligned, light passes through them uniformly. But turn one sheet by a few degrees, and the picture changes. Something similar happens with layers in certain materials: rotate them relative to each other, and their properties change — sometimes dramatically. Spectroscopy isn't the key here, or rather, it's not the method used.

Two sheets of graphene — carbon film one atom thick — without a twist resemble a harbor clogged with shoals: lithium ions either run aground or rush past without stopping. But a 9.43° twist lays out a perfect fairway. The ion ships dock securely to carbon 'piers' and immediately cast off without losing speed.

The crystal bismuth selenide (Bi₂Se₃) is symmetric by itself, like a neatly stacked sandwich: flip it over, and nothing changes. But twist one layer slightly relative to the other, and the symmetry breaks. The material begins to 'feel' which side the light comes from and responds with an electric current.

In layers of molybdenum telluride twisted at a specific angle, electrons behave unusually: they can split into parts. This is akin to a whole pie suddenly being cut into perfectly equal slices, even though there's only one knife. In ordinary materials, the electron's charge is indivisible, but in these layers, under strong cooling, states emerge where the charge splits into three parts.

A football kicked with spin doesn't fly straight: it bends to the side. A tightly focused beam of light does something similar: it starts pushing an atom not just forward but also sideways. Physicists measured this shift on a single calcium ion trapped in a light trap. The beam was aimed at the atom like a spotlight on a grain of sand.

An ordinary wave in a magnet is like ripples on water: the shorter the wavelength, the higher the energy. Physicists call such waves magnons. But in a thin film of bismuth yttrium iron garnet, they managed to create waves with the opposite behavior: their energy decreases as the wavelength gets shorter. These are antimagnons.

Light doesn't always travel in a straight line. Its wave can be twisted into a spiral, like a stream of water from a rapidly rotating hose. This twist is called orbital angular momentum, and it can hide information: each turn acts as a separate communication channel. The problem was that only bulky setups of lenses and mirrors could recognize these twists.

Throw pebbles along a tight row of posts: they ricochet into a dense spray. Throw at an angle, and they scatter. Gamma rays in a crystal do the same: they spawn particle cascades that spread out unless the crystal’s atomic rows are perfectly aligned. Align them within a hair’s width, and the cascade sharpens into a tight beam. A small crystal then outperforms a large one.

Dark matter is an invisible ocean that, as shown by observations by Vera Rubin, keeps stars in galaxies from flying apart. Physicists suspect that if dark matter consists of ultralight particles, in the cosmic cold it turns into a frictionless fluid — a so-called condensate.

In a typical magnet, all spins of electrons (internal ‘arrows’-magnets, discovered by Pauli) point one way. Altermagnets, like MnTe, are trickier: these arrows alternate direction, like ripples spreading from a pebble tossed into water. But here’s the twist — the ripple arises from the electrons’ own motion.