Physics professor David Weld receives Brown Investigator Award

Earlier this year, UC Santa Barbara physics professor, David Weld, was awarded the prestigious Brown Investigator Award. Established in 2020 through the Brown Science Foundation, the award supports curiosity-driven research fundamentals with the potential to seed breakthroughs that will benefit society. 

Through this award, Weld will receive $2 million in funding over the next five years to support his group’s research into an unusual and intriguing idea: controlling atoms to make a mirror that can exist in a quantum superposition of reflecting light and letting it pass through. 

The Weld lab cools down atoms such as potassium and lithium to one hundred-millionth of a degree above absolute zero, creating Bose-Einstein condensates — a state of matter where many atoms behave as though they are a single quantum object. At these ultracold temperatures, the atoms’ thermal motion slows down, allowing their quantum behavior and properties to become more pronounced and easier to study. Because the atoms in a condensate share the same quantum state, they can also behave collectively like a wave at these temperatures. 

The group aims to use these matter waves to control the atoms: by bringing multiple matter waves together, the waves can overlap and interfere with each other. According to Weld, the group intends to use atoms to make an atomic mirror, which, unlike a conventional mirror, would be intrinsically quantum mechanical.

“You can use those overlapping waves to build little structures of atoms,” Weld said. “It is in a quantum superposition state of reflecting all the light or reflecting none of the light.”

One way the group could create this superposition state is using Rydberg excitations, where an atom’s electron is moved to an orbital further away from the nucleus than normal. 

“Instead of orbiting the nucleus like Mercury, it’s orbiting like Pluto,” Weld explained. 

Moving the electron further from the nucleus greatly increases the atom’s size and polarisability, strengthening its interactions with neighboring atoms. 

“This lets you do very sharply defined quantum operations in these atomic gases,” Weld said. “That’s the kind of technique that we would pursue to try to create these superposition states.” 

The result would be a system in which both the light and matter that make up the mirror behave quantum mechanically — a system Weld described as “doubly quantum” optics. He described how the group could test the atomic mirror. 

“You could take one mirror and put it in a superposition state, then shine light off it and you should see that it all reflects or doesn’t reflect, but those things happen with equal probability,” Weld said. 

According to Weld, the group could take their investigations to the next level by arranging two mirrors in a loop — sending a single photon at this system could create quantum entanglement between the two mirrors.

“It actually projects the mirrors into what’s called a Bell pair,” Weld said. 

This phenomenon — in which the quantum states of two objects become connected so that measuring one reveals the state of the other — would further confirm that Weld’s group has created a quantum mechanical system that cannot be explained by classical physics. 

The potential applications of such a system extend to future quantum communication technologies. Optical fibers already play a central role in modern communication technology, carrying information as light over long distances by repeated total internal reflection, which keeps light contained within the fiber.

 “If you could do that in a way that the mirrors that guide around the light are also themselves quantum mechanical, I think that would open up a lot of possibilities that people haven’t really explored,” Weld said. 

The main driving force behind the Weld group’s research, however, is a fundamental curiosity to understand the quantum behaviour of these systems. 

Weld believes that “society actually benefits hugely from curiosity-driven research.” He recalled that decades ago, curiosity-driven research done into Gila monster venom eventually became essential to the development of GLP-1 drugs, a medication now taken by 10% of Americans for weight loss purposes. While this kind of research is not the kind with immediate and transparent benefits, such as elongating phone batteries, Weld believes that all types of research can “maximize practical human progress.”

The Brown Investigator Award will greatly contribute to the Weld lab in their investigations into producing a doubly quantum optic system. 

“I really want to express gratitude to the Brown Foundation for their support for curiosity-driven research, and to campus for their support to faculty,” Weld said.