Recent research has revealed that photon polarization is a topological property that remains constant in different environments, an insight that could improve fusion research by improving the design of the light beams used to heat the plasma. Credit: SciTechDaily
New studies show photon polarization is constant in different environments and potentially improves plasma methods of heating for the development of fusion energy.
Light, literally and figuratively, permeates our world. It dispels darkness, transmits telecommunications signals across continents, and reveals the unseen, from distant galaxies to microscopic bacteria. The light can also help heat the plasma in ring devices known as tokamaks, as scientists work to harness the fusion process to generate green electricity.
Recently, researchers at the Princeton Plasma Physics Laboratory discovered that one of the fundamental properties of photons—polarization—is topological, meaning that it remains constant even as the photon passes through different materials and environments. These findings, published in Physical overview D, could lead to more efficient plasma heating techniques and advances in fusion research.
Implications for fusion research
Polarization is the direction—left or right—that the electric fields move around the photon. Due to the fundamental laws of physics, the polarization of a photon determines the direction it travels and limits its path. As a result, a beam of light composed only of photons with a single type of polarization cannot spread to every part of a given space.
“A more precise understanding of the fundamental nature of photons could lead scientists to design better light beams for heating and measuring plasma,” said Hong Qin, principal research physicist at the US Department of Energy (DOE). PPPL and co-author of the study.
Artist’s rendering of photons, the particles that make up light, disrupting plasma. Credit: Kyle Palmer / PPPL Communications Department
Simplifying complex problems
Studying photons serves as a means of solving a larger, more difficult problem — how to use beams of intense light to induce long-lasting disturbances in plasma that could help maintain the high temperatures needed for fusion.
These waves, known as topological waves, often occur at the boundary of two different regions, such as the plasma and vacuum in tokamaks at their outer edge. They’re not particularly exotic—they occur naturally in Earth’s atmosphere, where they help produce El Niño, the gathering of warm water in the Pacific Ocean that affects weather in the Americas. To create these waves in plasma, scientists need to have a better understanding of light—specifically, the same kind of high-frequency waves used in microwave ovens—that physicists already use to heat plasma.
“We are trying to find similar waves for fusion,” Qin said. “They are not easily stopped, so if we could create them in the plasma, we could increase the efficiency of the plasma heating and help create the conditions for fusion.” The technique resembles ringing. Just as using a hammer to strike a bell causes the metal to move in such a way as to create sound, the researchers want to hit the plasma with light to vibrate in a certain way to create sustained heat.
Revealing the essence of the movement of photons
In addition to discovering that photon polarization is topological, the researchers discovered that the rotating motion of photons cannot be separated into intrinsic and extrinsic components. Think of the Earth: It both spins on its axis, creating day and night, and orbits the Sun, creating the seasons. These two types of movement usually do not affect each other; for example, the Earth’s rotation on its axis does not depend on its rotation around the Sun. In fact, the rotational motion of all material objects can be separated in this way.
However, it was not clear whether this was true for particles such as photons, which have no mass. “Most experimentalists assume that the angular momentum of light can be separated into spin and orbital angular momentum,” said Eric Palmerduca, lead author of the paper and a graduate student in Princeton’s program in plasma physics. “However, there has been a long debate among theorists about the proper way to do this division, or whether it is possible to do this division at all. Our work helps settle this debate, showing that photon angular momentum cannot be separated into spin and orbital components.
In addition, Palmerduca and Qin found that the two motion components cannot be separated due to topological, invariant properties of the photon, such as its polarization. This new finding has implications for the laboratory. “These results mean we need a better theoretical explanation of what’s going on in our experiments,” Palmerduca said.
These findings provide insight into the behavior of light and support the researchers’ goals of creating topological waves for fusion research.
Reviews for theoretical physics
Palmerduca notes that the photon findings demonstrate the power of PPPL in theoretical physics. The findings relate to a mathematical result known as the Hair Theorem. “The saying goes that if you have a ball covered in hair, you can’t brush all the hairs flat without creating a ball somewhere on the ball. Physicists thought that meant you couldn’t have a light source that was sending out photons in all directions at the same time,” Palmerduca said. However, he and Qin discovered that this is incorrect because the theorem does not take into account, mathematically, that photon electric fields can rotate.
The findings also add to the research of the former Princeton University Physics professor Eugene Wigner, whom Palmerduca identified as one of the most important theoretical physicists of the 20th century. Wigner realized that he could use principles derived from Albert Einstein’s theory of relativity to describe all possible elementary particles in the universe, even those that had not yet been discovered. But while his classification system is accurate for particles with mass, it gives inaccurate results for massless particles such as photons. “Qin and I have shown that using topology,” Palmerduca said, “we can modify Wigner’s classification for massless particles and provide a description of photons that works in all directions simultaneously.”
Future instructions
In future research, Qin and Palmerduca plan to explore how to create beneficial topological waves that heat the plasma without creating the useless variety that dissipates the heat. “Some harmful topological waves can be excited unintentionally, and we want to understand them so we can remove them from the system,” Qin said. “In this sense, topological waves are like new breeds of insects. Some are beneficial to the garden and some are pests.”
Meanwhile, they are excited about the current findings. “We have clearer theoretical knowledge about photons that could help excite topological waves,” Qin said. “Now is the time to build something so we can use them in our search for fusion energy.”
Reference: “Photon topology” by Eric Palmerduc and Hong Qin, 2 Apr 2024, Physical overview D.
DOI: 10.1103/PhysRevD.109.085005