Scientists have discovered that the polarisation of photons is topological; it doesn’t change even as the photon moves through different materials and environments.
The discovery was made as part of nuclear fusion research carried out at the Princeton Plasma Physics Laboratory (PPPL). The team was investigating how beams of intense light can excite long-lasting perturbations in plasma, which could help maintain the high temperatures needed for nuclear fusion.
Since a photon’s polarisation helps determine the direction the photon travels and limits its movement, a beam of light made up of only photons with one type of polarisation cannot spread into every part of a given space.
“Having a more accurate understanding of the fundamental nature of photons could lead to scientists designing better light beams for heating and measuring plasma,” said Hong Qin, a principal research physicist at the US Department of Energy’s (DOE) PPPL.
Creating topological waves in plasma for fusion
In nuclear fusion, light can help heat the plasma within ring-shaped devices known as tokamaks as scientists worldwide strive to harness the fusion process to generate green electricity.
Though the PPPL researchers were studying individual photons, they were doing so as a way to solve a larger, more difficult problem. Known as topological waves, these ‘wiggles’ often occur on the border of two different regions, like plasma and the vacuum in tokamaks at its outer edge. They are not especially exotic – they occur naturally in Earth’s atmosphere, where they help produce El Niño, a gathering of warm water in the Pacific Ocean that affects weather in North and South America.
“We are trying to find similar waves for fusion,” said Qin. “They are not easily stopped, so if we could create them in plasma, we could increase the efficiency of plasma heating and help create the conditions for fusion.” The technique resembles ringing a bell. Just as using a hammer to hit a bell causes the metal to move in such a way that it creates sound, the scientists want to strike plasma with light so it wiggles in a certain way to create sustained heat.
The angular momentum of photons cannot be split into spin and orbital components
In addition to discovering that a photon’s polarisation is topological, the scientists found that the spinning motion of photons could not be separated into internal and external components. Similarly, Earth both spins on its axis, producing day and night, and orbits the sun, producing the seasons. These two types of motion typically do not affect each other; for instance, Earth’s rotation around its axis does not depend on its revolution around the sun. In fact, the turning motion of all objects with mass can be separated this way.
But scientists have not been so sure about particles such as photons, which do not have mass. “Most experimentalists assume that the angular momentum of light can be split into spin and orbital angular momentum,” said said Eric Palmerduca, a graduate student at the PPPL. “However, among theorists, there has been a long debate about the correct way to do this splitting or whether it is even possible to do this splitting. Our work helps settle this debate, showing that the angular momentum of photons cannot be split into spin and orbital components.”
Moreover, it was established that the two movement components can’t be split because of a photon’s topological, unchanging properties, like its polarisation. This novel finding has implications for the laboratory. “These results mean that we need a better theoretical explanation of what is going on in our experiments,” Palmerduca said.
All of these findings about photons give the researchers a clearer picture of how light behaves. With a greater understanding of light beams, they hope to figure out how to create topological waves that could be helpful for fusion research.
Insights for theoretical physics
The findings relate to a mathematical result known as the Hairy Ball Theorem. “The theorem states that if you have a ball covered with hairs, you can’t comb all the hairs flat without creating a cowlick somewhere on the ball. Physicists thought this implied that you could not have a light source that sends photons in all directions at the same time,” Palmerduca said. He and Qin found, however, that this is not correct because the theorem does not take into account, mathematically, that photon electric fields can rotate.
The findings also amend research by former Princeton University Professor of Physics Eugene Wigner, who Palmerduca described as one of the most important theoretical physicists of the 20th century. Wigner realised that using principles derived from Albert Einstein’s theory of relativity, he could describe all the possible elementary particles in the universe, even those that hadn’t been discovered yet. But while his classification system is accurate for particles with mass, it produces inaccurate results for massless particles, like photons.
“Qin and I showed that using topology, we can modify Wigner’s classification for massless particles, giving a description of photons that work in all directions at the same time," said Palmerduca.
A clearer understanding for the future
In future research, Qin and Palmerduca plan to explore how to create beneficial topological waves that heat plasma without making unhelpful varieties that siphon the heat away. “Some deleterious topological waves can be excited unintentionally, and we want to understand them so that they can be removed from the system,” Qin said.
“In this sense, topological waves are like new breeds of insects. Some are beneficial for the garden, and some of them are pests.”
Meanwhile, they are excited about the current findings. “We have a clearer theoretical understanding of the photons that could help excite topological waves,” Qin said. “Now it’s time to build something so we can use them in the quest for fusion energy.”
The research has recently been published in Physical Review D.
