In a groundbreaking advancement that unlocks new possibilities for the future of quantum and classical information technologies, researchers at the Okinawa Institute of Science and Technology (OIST) have, for the first time, directly observed the evolution of dark excitons in atomically thin materials. This achievement marks a significant milestone in the long-standing quest to exploit these elusive quasiparticles as robust carriers of quantum information. Published in the prestigious journal Nature Communications in July 2025, this research not only illuminates the mysterious behavior of dark excitons but also paves the way towards leveraging their unique properties for next-generation devices.
Excitons, fundamental to the operation of semiconductors, arise when electrons absorb energy and leap into a higher band structure, leaving behind holes in their previous energy levels. These electron-hole pairs are bound by electrostatic forces and behave collectively as quasiparticles. Within this realm, excitons fall into two categories: bright and dark. Bright excitons, characterized by matching quantum properties such as spin and momentum (or valley states), recombine swiftly and emit photons, thereby interacting strongly with light. In contrast, dark excitons possess mismatched quantum attributes that forbid immediate recombination, rendering them invisible to light but endowing them with longer lifetimes and remarkable isolation from environmental perturbations.
The study of dark excitons has been challenging precisely because their invisibility to conventional optical techniques has made them difficult to detect and manipulate. Yet, their potential as carriers of quantum information is immense due to their resistance to decoherence—a common problem where quantum information is lost to environmental noise. Professor Keshav Dani, leading the Femtosecond Spectroscopy Unit at OIST, emphasizes this potential, remarking that the inherent darkness of these excitons shields their quantum states from degradation, a quality that could revolutionize how information is processed and stored in future technologies.
The team’s exploration delves into a cutting-edge arena known as valleytronics, where the valley degree of freedom—the distinct momentum states electrons occupy in the crystal lattice—serves as a new information channel. This paradigm extends beyond conventional electronics, which manipulates charge, and spintronics, which manipulates electron spins. Valleytronics exploits the unique crystal symmetry and electronic band structure of transition metal dichalcogenides (TMDs), a class of two-dimensional materials that have garnered extensive attention for their extraordinary electronic and optical properties.
TMDs, such as monolayer tungsten disulfide (WS2), exhibit multiple valleys in their momentum space, each acting as a potential ‘bucket’ to encode information. When illuminated with circularly polarized light, bright excitons are selectively generated in specific valleys, setting the stage for valley-dependent phenomena. However, these bright excitons rapidly scatter into numerous dark excitons, which, although optically silent, could potentially retain valley information over significantly longer timescales and thus serve as superior information carriers.
The complexity of these excitonic states increases as they include two main types of dark excitons: momentum-dark and spin-dark. Momentum-dark excitons arise when electrons and holes occupy mismatched valleys in momentum space, prohibiting recombination due to momentum conservation laws. Spin-dark excitons occur when spins of electron and hole are antiparallel, preventing radiative recombination even when co-located in momentum space. Both species exhibit lifetimes extending from a few picoseconds to several nanoseconds, vastly outlasting bright excitons and offering a tantalizing temporal window for quantum information operations.
To dissect the intricate dance of these excitons over time and space, the team employed the state-of-the-art time- and angle-resolved photoemission spectroscopy (TR-ARPES) setup, uniquely equipped with a custom-built extreme ultraviolet (XUV) light source. This sophisticated technique enables simultaneous measurement of electron momentum, spin states, and population dynamics with femtosecond resolution. By directly capturing the ultrafast dynamics across multiple excitonic species in monolayer WS2, the researchers overcame the fundamental challenge of dark exciton invisibility, providing an unprecedented holistic view of valley-polarized excitonic behavior at the quantum level.
The experimental results unveiled a vivid timeline of excitonic transformations. Initially, bright excitons created in targeted valleys via polarized light were observed to scatter within a handful of picoseconds through interactions with phonons—quantized vibrations in the lattice—transitioning into momentum-dark excitons located in different valleys. Subsequently, spin-flip processes led to the emergence of spin-dark excitons that dominated the landscape over nanosecond lifetimes. This gradual evolution signifies a natural progression from bright to long-lived dark excitons that preserve valley polarization, crucial for their potential application in information processing.
This discovery carries profound implications for the development of quantum information systems. The longevity and environmental resilience of dark excitons make them ideal quantum bits (qubits) that could operate under less stringent conditions than current qubit technologies, which typically require extreme cooling and isolation. Unlike bright excitons, which rapidly lose coherence due to their strong interaction with light and environment, dark excitons’ “invisibility” grants them a protective cloak that could facilitate durable quantum states essential for computation and communication.
Breaking new ground in dark valleytronics, this research lays fertile ground for technologies that exploit these dark excitons to encode, manipulate, and read quantum information. As Dr. Julien Madéo from the OIST Femtosecond Spectroscopy Unit notes, the capability to directly access and monitor dark exciton states will stimulate innovative approaches towards integrating these quasiparticles into practical devices, thereby bridging the gap between fundamental quantum phenomena and scalable technology platforms.
Future endeavors will focus on developing methods to efficiently read out the valley information encoded in dark excitons, a critical step to harness their full potential. This could involve refined optical or electrical probing techniques that circumvent their natural invisibility, enabling real-time control and utilization in quantum circuits. The ongoing collaboration of material scientists, spectroscopists, and quantum engineers aims to translate these fundamental findings into robust, versatile, and commercially viable quantum devices.
This remarkable achievement underscores the transformative power of combining atomically precise materials engineering with advanced ultrafast spectroscopy. By unveiling the hidden quantum landscapes of dark excitons and elucidating their dynamic evolution, the research from OIST’s Femtosecond Spectroscopy Unit not only expands the frontiers of condensed matter physics but also charts a compelling roadmap towards future quantum information technologies that leverage the untapped potential of dark valley physics.
Subject of Research:
Not applicable
Article Title:
A holistic view of the dynamics of long-lived valley polarized dark excitonic states in monolayer WS2
News Publication Date:
10-Jul-2025
Web References:
https://doi.org/10.1038/s41467-025-61677-2
References:
Okinawa Institute of Science and Technology (OIST), Nature Communications, 2025
Image Credits:
Jeff Prine (OIST)
Keywords:
dark excitons, valleytronics, transition metal dichalcogenides, monolayer WS2, TR-ARPES, quantum information technologies, spintronics, phonons, femtosecond spectroscopy, quantum coherence, nanosecond lifetimes, quantum qubits
