How can we better hold environmental polluters accountable? How can we enhance the efficiency of qubits? These questions, which loom large for the researchers who study them, are the type of big-issue topics that UC Santa Barbara graduate students are encouraged to tackle. And they’re the central themes of the dissertations that won the 2021-2022 Winifred and Louis Lancaster Dissertation Awards.

This year’s recipients are Emily Williams and Mark Turiansky selected by the awards committee for dissertations with “significant impact on the field in terms of methodological and substantive contributions.”

Quantum Mechanic
The quantum world holds much potential for those who learn to wield it. This space of subatomic particles and their behaviors, interactions and emergent properties can open the door to new materials and technologies with capabilities we have yet to even dream of.

Mark Turiansky, a PhD student advised by materials professor Chris Van de Walle, is among those at the forefront of this discipline at UCSB, joining some of the finest minds in the quantum sciences as a fellow at the NSF-supported UCSB Quantum Foundry.

“The field of quantum information science is rapidly developing and has garnered a ton of interest,” said Turiansky, who developed an abiding interest in physics as a child. “In the past few years, billions of dollars of funding have been allocated to quantum information science.”

Enabled by relatively recent technologies that allow for the study of the universe at its smallest scales, quantum researchers like Turiansky are still just scratching the surface as they work to nail down the fundamentals of the strange yet powerful reality that is quantum physics.

At the heart of some of these investigations is the quantum defect — imperfections in a semiconductor crystal that can be harnessed for quantum information science. One common example is the nitrogen-vacancy center in a diamond: In an otherwise uniform crystalline carbon lattice, an NV center is a defect wherein one carbon atom is replaced with a nitrogen atom, and an adjacent spot in the lattice is vacant. These defects can be used for sensing, quantum networking and long-range entanglement.

The NV center is only one such type of quantum defect, and though well-studied, has its limitations. For Turiansky, this underlined the need to gain a better understanding of quantum defects and to find ways to predict and possibly generate more ideal defects.

These needs became the basis of his dissertation, “Quantum Defects from First Principles,” an investigation into the fundamental concepts of quantum defects, which could lead to the design of a more robust qubit — the basic unit of a quantum computer.

To explore his subject, Turiansky turned his attentions to hexagonal boron nitride.

“Hexagonal boron nitride is an interesting material because it is two-dimensional,” he explained, “which means that you can isolate a plane of the material that is just one atom thick.” By shining light on this material, it is possible to detect quantum defects called “single-photon emitters” by the bright spots that shine back. These single photons, he added, are “inherently quantum objects that can be used for quantum information science.”

“The main feat was identifying the defect that was responsible for single-photon emission,” Turiansky said. He accomplished it with computational methodologies that he worked to develop in his research.

“One methodology that I’ve worked on a lot is for nonradiative recombination,” he said, describing it in his paper as “fundamental to the understanding of quantum defects, dictating the efficiency and operation of a given qubit.” By applying his methodology, Turiansky was able to determine the origin of these single photon emitters — a topic of much debate in the community. It’s a feat that could be applied to examine other quantum defects, and one that was deemed worthy of the Lancaster Award.

“Mark’s work has moved the field forward by systematically identifying promising quantum defects, and providing an unambiguous identification of the microscopic nature of the most promising quantum emitter in hexagonal boron nitride,” remarked Turiansky’s adviser, materials professor Chris Van de Walle. “He accomplished this by creatively applying the computational approaches he developed and fruitfully collaborating with experimentalists.”

“It’s really an exceptional honor to receive such a prestigious award for my research efforts over the last five years,” Turiansky said. “It’s even more meaningful knowing the high quality of research turned out at UCSB and the fierce competition of my peers. I’m incredibly grateful to my adviser, group members, collaborators, friends and family who helped make this achievement possible.”

To read more about Williams and her project, click here to read the article published in The Current. 

The two Lancaster dissertations are enteres into a national competition sponsored by the Council of Graduate Schools. A check for $1,000 and a plaque will be awarded upon completion of entry for the national competition.