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

Dissipative phase transitions, often characterized by energy loss to the surrounding environment, occur in various physical systems and can lead to substantial changes in those systems’ states. Such transitions are of particular interest in quantum mechanics, where phenomena like entanglement and superposition defy classical intuition. The study of DPTs is crucial as they encompass both first-order transitions, likened to flipping a switch, which engender abrupt changes in the state of a system, as well as second-order transitions that, while more continuous, challenge the limits of symmetry in quantum physics.

One of the essential challenges in understanding DPTs has been the ability to measure them accurately, especially the second-order transitions, which have eluded observation due to their subtle characteristics. This research hinges on creating a controlled environment capable of minimizing noise and maximizing the sensitivity of measurements. The team’s innovative use of a Kerr resonator—a device that enhances and amplifies minute quantum effects—was pivotal in facilitating unprecedented observations of phase transitions with a level of detail that traditional setups could not provide.

The experimental approach by Scarlino’s team involved tuning the resonator’s parameters—specifically its detuning and drive amplitude—to systematically force the system through different quantum states. This meticulous process allowed the researchers to witness both first- and second-order DPTs directly. Remarkably, the team demonstrated how, through controlled energy input via a two-photon drive, they could fine-tune the resonator’s conditions to study its transition behaviors accurately.

Validation of these phase transitions was achieved through experimentation at temperatures approaching absolute zero. Operating in this regime drastically reduced extraneous thermal noise, enabling the researchers to isolate and observe the Kerr resonator’s dynamics without interference. As a result, the experiment not only showcased the capability to observe quantum states but also amplified phenomena typically drowned out by environmental factors.

One of the striking observations made by the team was the phenomenon known as “squeezing” during the second-order DPT. In such cases, quantum fluctuations fell below the natural noise level of empty space, indicating that the system had entered a transformative state marked by extreme sensitivity to changes in control parameters. This creates an elegant interplay between the observable quantum properties and the underlying thermodynamic principles that govern behavior in such complex systems.

In addition to the second-order transitions, the first-order DPT revealed distinct hysteresis cycles, illustrating how transitions could depend on the system’s history. Such hysteresis is indicative of a system influenced by competing phases and can lead to noteworthy implications for the stability and control of quantum devices. Understanding these hysteresis cycles is essential for engineers and physicists who aim to design resilient quantum systems.

Crucially, both types of transitions demonstrated evidence of critical slowing down—a universally shared phenomenon near critical points where the system’s response time increases substantially as the transition approaches. This slowing down not only reinforces the predictions made using Liouvillian theory but also hints at utilizing these traits to develop more nuanced quantum measurement techniques.

The implications of this research extend into potential applications, particularly in the realm of quantum-computing technologies. By harnessing the insight provided by understanding DPTs, future quantum computers may achieve more robust error correction capabilities and improved stability amidst noise. The excitement surrounding this capability is palpable among physicists as they anticipate the fusion of theoretical insights with practical applications.

At its core, this study celebrates the synergy between theoretical and experimental physics. The collaborative effort of research institutions, including Sapienza University, Aalto University, and the University of Pavia, underscores how interdisciplinary cooperation can lead to advancements in quantum science that were previously deemed unattainable.

Guillaume Beaulieu, the lead author of the study, aptly described the joint efforts that facilitated these findings: “In fact, a very interesting aspect of this work is that it also demonstrates how close collaboration between theory and experiment can lead to results far greater than what either group could have achieved independently.”

As quantum research continues to gain momentum, revealing secrets of the microscopic world, the study’s outcomes serve as a stepping stone toward unraveling new phenomena, ultimately enhancing our ability to manipulate and utilize quantum systems effectively. The seamless blending of advanced engineering, rigorous experimentation, and profound theoretical insights heralds a new era in quantum physics, poised to influence technology and our understanding of fundamental nature.

By unraveling the mysteries surrounding dissipative phase transitions, this team has firmly positioned itself at the forefront of quantum research, opening exciting new avenues for inquiry and innovation in quantum mechanics. The ripple effects of these findings will certainly influence the landscape of future quantum technologies.

Subject of Research: Dissipative Phase Transitions in Quantum Systems
Article Title: Observation of first- and second-order dissipative phase transitions in a two-photon driven Kerr resonator.
News Publication Date: 10-Mar-2025
Web References: Nature Communications Article
References: Beaulieu, G., Minganti, F., Frasca, S., Savona, V., Felicetti, S., Di Candia, R., & Scarlino, P. (2025). Observation of first- and second-order dissipative phase transitions in a two-photon driven Kerr resonator. Nature Communications. DOI: 10.1038/s41467-025-56830-w
Image Credits: Guillaume Beaulieu (EPFL)

Keywords: Quantum Phase Transitions, Dissipative Phase Transitions, Quantum Computing, Kerr Resonator, Quantum Mechanics, Quantum Information, Superconductivity, Experimental Physics.