If the electron and hole share matching quantum properties, such as the same spin and momentum valley, they recombine quickly and emit light. These are bright excitons<\/a>. But if their properties clash, they\u2019re forbidden from recombining, and no light is emitted. These are dark excitons, invisible to traditional detection methods but potentially powerful as information carriers.<\/p>\n Professor Keshav Dani, who heads the spectroscopy unit at OIST, emphasized the importance of these shadowy particles. He noted that dark excitons are naturally resistant to light-induced degradation, making them ideal for preserving quantum states.<\/p>\n \u201cTheir invisibility makes them hard to study,\u201d he acknowledged, \u201cbut also gives them a unique advantage in maintaining coherence, an essential trait for quantum technologies.\u201d<\/p>\n Building on a 2020 breakthrough, Dani\u2019s team has now developed a method to create, observe, and manipulate dark excitons, opening a new frontier in quantum research.<\/p>\n New quantum state discovered at the intersection of exotic materials<\/a><\/p>\n The study also touches on the emerging field of valleytronics, where information is encoded not in charge or spin, but in the momentum valleys of electrons within a crystal lattice.<\/p>\n PhD researcher Xing Zhu, co-first author of the study, explained the progression: \u201cIn electronics, we use charge. In spintronics, we use spin. In valleytronics, we go further, using the crystal\u2019s structure to encode data in momentum states.\u201d<\/p>\n Dark excitons are excellent candidates for valleytronics. Unlike traditional qubits, they\u2019re more resistant to environmental noise and may require less extreme cooling, making them more practical for real-world quantum devices.<\/p>\n Dr. David Bacon, now at University College London, helped classify the two types of dark excitons: momentum-dark and spin-dark. Momentum-dark excitons arise when electrons scatter into different valleys, while spin-dark excitons result from spin mismatches within the same valley.<\/p>\n \u201cThese mismatches prevent recombination,\u201d Bacon explained, \u201callowing dark excitons to persist for nanoseconds, a much more useful timescale for quantum operations.\u201d<\/p>\n To observe these fleeting particles, the team used OIST\u2019s cutting-edge TR-ARPES system (time- and angle-resolved photoemission spectroscopy), powered by a proprietary extreme ultraviolet (XUV) source. This setup enabled them to simultaneously measure the momentum, spin, and population levels of electrons and holes, something that had never been achieved before.<\/p>\n Dr. Vivek Pareek, now a Presidential Postdoctoral Fellow at Caltech, highlighted the role of circularly polarized light in selectively creating bright excitons in specific valleys. \u201cThese bright excitons quickly convert into dark ones,\u201d he said. \u201cUnderstanding which species preserve valley information is crucial for future valleytronic applications.\u201d<\/p>\n The team\u2019s observations revealed a dynamic evolution: within a picosecond, bright excitons scatter into momentum-dark states via the emission of phonons. Over time, spin-dark excitons dominate, lingering for nanoseconds and preserving valley information.<\/p>\n Dr. Julien Mad\u00e9o, a senior member of the unit, summarized the impact: \u201cThanks to our TR-ARPES system, we\u2019ve mapped how dark excitons retain valley information. This lays the groundwork for dark valleytronics, a field that could revolutionize how we store and process quantum data.\u201d<\/p>\n This pioneering work not only illuminates a previously hidden corner of quantum physics but also opens the door to practical, scalable quantum systems. By harnessing the stability and longevity of dark excitons, researchers may soon build devices that are faster, cooler, and more resilient than anything we\u2019ve seen before.<\/p>\n Journal Reference:<\/strong><\/p>\n![\"different](\"https:\/\/www.europesays.com\/ie\/wp-content\/uploads\/2025\/10\/different-exciton-emerge-1024x584.webp.webp\"\/)
Graphical illustration of the results, showing how the population of different exciton emerge and evolve over time at a picosecond scale (1ps = 10\u221212 second).
\nJack Featherstone (OIST), adapted from Zhu et al. (2025) Nature Communications 16 6385.<\/p>\n\n