Using advanced spectroscopy techniques, researchers at the Okinawa Institute of Science and Technology (OIST) observed the evolution of dark excitons in tungsten disulfide (WS2), an atomically-thin semiconductor from the transition metal dichalcogenide (TMD) family of materials. The researchers’ discovery lays important groundwork for the future use of dark valleytronics technology in quantum technologies. Valleytronics uses the valley position of electrons to process information. Dark excitons, with their long lifetimes and resistance to decoherence, are more resistant to environmental factors such as thermal background than the current generation of qubits, and may also may require less cooling, making them less susceptible to decoherence. In effect, the work showed that, in monolayer semiconductors, dark excitons have the potential to enable new opportunities in quantum technologies: The valley dimension of dark excitons could be used to carry quantum information. Experimental equipment used for dark exciton research. Courtesy of OIST/Jeff Prine. The researchers used the time- and angle-resolved photoemission spectroscopy setup at OIST, called TR-ARPES, to holistically track the characteristics of all excitons in the TMD material, after the creation of bright excitons in a specific valley in the material, over time. The researchers simultaneously quantified momentum, spin state, and population levels of the electrons and holes in the TMD. They found that within 1 ps, phonons scattered some bright excitons into different momentum valleys, causing the bright excitons to become momentum-dark. Later, spin-dark excitons, a state where electrons have flipped their spin within the same valley, dominated, and persisted on nanosecond scales. “There are two ‘species’ of dark excitons — momentum-dark and spin-dark, depending on where the properties of electron and hole are in conflict,” researcher David Bacon said. In the TMD, the momentum-dark excitons largely dominated at early times, sustaining a 40% degree of valley polarization, while the valley-polarized spin-dark states dominated at longer times. The experimental setup at OIST, using the world-leading time- and angle-resolved photoemission spectroscopy (TR-ARPES) microscope, which features a proprietary, tabletop extreme ultraviolet (EUV) source, capable of imaging the electrons and excitons at femtosecond timescales. Courtesy of OIST/Jeff Prine and Andrew Scott. The TMD material, through its specific spin- and energy-ordering of excitons, plays a role in the transfer of valley polarization from the bright to the momentum-dark excitons. "The unique atomic symmetry of TMDs means that when exposed to a state of light with a circular polarization, one can selectively create bright excitons only in a specific valley. This is the fundamental principle of valleytronics,” researcher Vivek Pareek said. When the negatively-charged electrons in the TMD are exposed to light, the electrons are excited to a higher energy state, leaving behind a positively-charged hole in the valence band. The electrons and holes are bound together, forming excitons. If certain quantum properties of the electron and hole match — that is, if they have the same spin configuration and they inhabit the same valley in momentum space, they recombine, emitting light in the process. These excitons are characterized as bright excitons. If the quantum properties of the electron and hole do not match, the electron and hole are prevented from recombining on their own, and do not emit light. These are characterized as dark excitons. “The mismatch in properties not only prevents immediate recombination, allowing them to exist up to several nanoseconds, but also makes dark excitons more isolated from environmental interactions,” Bacon said. “Dark excitons have great potential as information carriers, because they are inherently less likely to interact with light, and hence less prone to degradation of their quantum properties,” professor Keshav Dani said. The invisible nature of dark excitons makes them challenging to study and manipulate. However, the team has overcome the fundamental challenge of how to access and track dark excitons, setting the stage for dark valleytronics as a field. “Bright excitons rapidly turn into numerous dark excitons that can potentially preserve the valley information,” Pareek said. “Which species of dark excitons are involved and to what degree they can sustain the valley information is unclear, but this is a key step in the pursuit of valleytronic applications.” According to researcher Julien Madéo, future developments aimed at reading out the dark excitons’ valley properties will now unlock broad dark valleytronic applications spanning information systems. The research was published in Nature Communications (www.doi.org/10.1038/s41467-025-61677-2).