Quantum mechanics continues to be a frontier for exploration, offering a wealth of possibilities for technological advancements. In recent years, the study of excitons in organic semiconductors—mobile quasiparticles formed by quantum states with electrons in higher energy levels—has gained momentum. Among the materials drawing attention in this field is rubrene, an organic semiconductor with unique properties that make it an excellent candidate for fundamental research into certain quantum properties of excitons. Physics Ph.D. student Jace Curran studies the fundamental properties of these excitons in rubrene, gaining valuable insights into this evolving area of study.

At the core of Curran’s research lies the behavior of triplet excitons in rubrene. These excitons, which form from photoexcited states through a process called singlet fission, exhibit spin entanglement—a quantum mechanical property where two particles are interdependent, even when separated. This interdependence ultimately leads to a process of quantum interference when two excitons recombine to emit a photon, and this interference in turn leads to a time-modulation of the light emitted by the triplet excitons. These oscillations in fluorescence intensity are called quantum beats, and they are a unique consequence of spin-entanglement in pairs of excitons. Curran investigates these quantum beats and their origin and uses them to characterize the behavior of triplet excitons in rubrene. 

“We create these excitons in this spin-entangled state, and their entanglement persists even as they move independently around the crystal,” he says. “By studying when and why this spin entanglement breaks down, we gain insight into the quantum mechanical processes governing their behavior.”

This work is significant for its possible implications in technologies such as quantum computing and solar energy. Triplet excitons that can be entangled could serve as qubits, the fundamental units of quantum computers, while the efficient generation of excitons by the singlet fission process could enhance the efficiency of solar cells by providing two electrons in an external circuit for the price of one photon.

The Experimental Setup

Working in the laboratory of quantum physicist Ivan Biaggio, Curran’s experiments involve the precise use of lasers to excite rubrene crystals. The Biaggio lab focuses on understanding the dynamics of exciton behavior, measuring how they evolve and interact over time.

“We use a picosecond pulse to excite the system, observe quantum beat oscillations over nanoseconds, and watch the triplets decay over microseconds,” Curran says. “After 10 microseconds or so, our triplets have all decayed, and we’re back in the ground state. We can come in with the next laser pulse and repeat the process again. By isolating quantum beats from other fluorescence signals, we analyze how spin entanglement evolves.”

Rubrene's favorable properties make it an ideal subject for such studies. Its high carrier mobility, ease of fluorescence, and alignment of crystalline axes simplify experimentation and data collection. Rubrene crystals are grown using a process called vapor transport. In this method, rubrene molecules are heated until they turn into vapor inside a tube filled with argon gas. As the tube cools down gradually, the vapor molecules stick to the tube walls and form solid crystals. These crystals have a specific shape and structure, with their molecules packed in a repeating pattern called an orthorhombic lattice.

What makes rubrene particularly interesting is that this uniform molecular configuration allows scientists to apply external forces, like magnetic fields or fluorescence, in a way that affects all the molecules in the same way. This simplifies experiments, such as studying how rubrene glows when hit with a pulse of light while under a magnetic field. This isn't possible with similar materials, like tetracene, because their molecules don’t align as neatly.

Addressing Unexplained Phenomena

One of the more intriguing findings in Curran’s research involves the unexpected absence of quantum beats under certain conditions. These instances, which do not align with existing theoretical models, were the focus of a recent article he authored.

“When we apply magnetic field, for certain orientations of field, we don't get quantum beats. And that’s not really explained or accounted for in the quantum mechanics,” he says.

By addressing these anomalies, Curran contributes to a deeper understanding of quantum systems, potentially paving the way for more robust quantum technologies. The implications of this work extend beyond academia. Though theoretical, Curran emphasizes the potential for possibly using spin-entangled triplet excitons in fields such as quantum computing.

Curran’s journey into this field is as unique as the research itself. Originally a game design major, he transitioned to physics after a stint in corporate America. This unconventional path reflects a determination to tackle the most challenging of academic disciplines.

“Physics seemed like a difficult, yet rewarding path,” he says. “It's kind of understanding, at the most molecular, fundamental level, how the universe operates.”

This approach mirrors the resilience required to probe the intricacies of quantum mechanics, where challenges often outnumber answers.

Curran’s work with rubrene exemplifies the blend of curiosity, persistence, and innovation that drives scientific discovery. By unraveling the complexities of quantum mechanics and exploring the applications of excitons, he contributes to a field with profound implications for technology and sustainability. As the study of excitons continues to evolve, researchers like Curran are pushing the boundaries of what is possible, forging pathways to a quantum-powered future.

As Curran nears the completion of his doctoral program, he remains optimistic about the future of exciton research. While considering a career in either industry or academia, he is keen to publish one final paper addressing global decoherence and its effects on spin-entangled states.

“I'm lucky to be part of a big, extended team that's all pretty great at what they do,” he says. “There's a lot more to pull out from this, and because we have this system partially solved, and because we know how to work with it and it is so easy to work with it, there's stuff to be had here for the next five to 10 years, depending on who comes into the group and who wants it. There’s so much more science to be had here.”