Why this physicist is bringing thermodynamics to the quantum age

Thermodynamics, developed in the 1800s in the context of the Industrial Revolution, describes physics concepts of heat, work and energy (SN: 6/12/24). The field was born from scientific efforts to understand steam engines. In contrast to those clanging and banging industrial machines, quantum physics describes phenomena on the scale of atoms, electrons and the like, and has driven the development of modern technologies such as quantum computers (SN: 6/28/23).

In the past, some physicists didn’t think that the idea of quantum thermodynamics made sense. “They saw it as an oxymoron,” Yunger Halpern says.

Now, though, the two concepts collide in quantum engines and other miniature devices. Quantum thermodynamics researchers aim to develop the tools to describe heat, work, cooling and efficiency in quantum systems, and determine the limits of performance of quantum devices. Yunger Halpern, a National Institute of Standards and Technology physicist based at the Joint Center for Quantum Information and Computer Science in College Park, Md., is at the forefront of those efforts.

“She has a vision, and she follows it,” says quantum physicist Aram Harrow of MIT. “She is good, also, at recruiting other people to her vision.”

One of Yunger Halpern’s major contributions has been exploring what the quantum concept behind Heisenberg’s uncertainty principle might mean for thermodynamics.

Picture a cup of hot tea. Thermodynamics describes how energy moves from the tea to the surrounding air, or how evaporating water molecules escape. Both of those quantities — energy and water molecules — are conserved in this scenario, meaning that they can move from one place to another, but the total amount is fixed. The problem of explaining how conserved quantities are exchanged shows up repeatedly in thermodynamics.

Now, what if the tea wasn’t an entire cup but a bundle of just a few atoms? Yunger Halpern wants to know how the exchange would differ. In quantum physics, conserved quantities can be incompatible with one another. That means they can’t be measured simultaneously. Heisenberg’s uncertainty principle, which states that the better you know the position of a quantum object, the worse you know its momentum, and vice versa, provides a famous example (SN: 1/12/22).

“For many decades, almost no one really thought about what happens when you have a system and environment that exchange quantities that are incompatible,” Yunger Halpern says. It turns out that incompatibility can have a real impact on how the system behaves, she and colleagues noted in a survey of the topic published in 2023 in Nature Reviews Physics. For example, incompatibility can decrease the amount of entropy, or disorder, that’s produced in such exchanges. Because the total entropy of an isolated system tends to increase over time,  some scientists think that entropy is closely related to an “arrow of time” that distinguishes future from past (SN: 7/10/15). In some sense, Yunger Halpern says, that means incompatible quantities might hinder a system’s ability to experience that arrow of time.

Quantum thermodynamics has led to some neat laboratory demonstrations. For example, a single atom can be made into a quantum engine that converts heat into work (SN: 4/14/16). Now, Yunger Halpern aims to put quantum thermodynamics to practical use through autonomous quantum machines.

Typical quantum devices, such as single-atom engines, atomic clocks or the quantum bits that make up quantum computers, require constant prodding from experimenters to operate. Autonomous devices would operate automatically.

Yunger Halpern teamed up with colleagues to bring this idea into reality. The result was an autonomous quantum refrigerator that can automatically cool a quantum bit, the team reported in May 2023 at arXiv.org.

And in a July 2023 arXiv paper, she and colleagues laid out criteria that must be met to create an autonomous quantum machine. For example, these machines must have structural integrity and sufficiently pure quantum states. In addition, their output must be worth the input needed to run them. This means a quantum engine can’t take more energy to control it than it outputs. Quantum physicist Marcus Huber worked with Yunger Halpern on developing those criteria. “I found her brilliant, but also mega-intense and focused,” says Huber, of TU Wien in Vienna. “She’ll barrage you with pertinent and good questions.”

It’s not just her science that’s in the spotlight — her writing is too. Yunger Halpern’s 2022 book, Quantum Steampunk: The Physics of Yesterday’s Tomorrow, drew public attention to the field. She’s also a science blogger at the website Quantum Frontiers. Writing, Yunger Halpern says, allows her to explore fresh ideas without the constraints of scientific publications (fanciful speculation and “out-there” ideas aren’t likely to pass peer review). “Thinking really broadly and wildly and as creatively as you feel like thinking on any given day of the month is useful for keeping creative in physics.”

And just as her work juxtaposes old and new, Yunger Halpern often exemplifies contrasts, says Shayan Majidy of the University of Waterloo in Canada and soon to join Harvard University, who recently completed his Ph.D., coadvised by Yunger Halpern. She holds her students to high standards but is warm and caring as an adviser. Majidy says that when he got married, Yunger Halpern somehow figured out his favorite local brand of ice cream — Kawartha Dairy— and sent him a gift card.

Her hobbies tend toward quiet, slow-paced pursuits: going for walks, visiting museums. Yet she injects vigorous passion into her work. “She has very old-fashioned interests and taste,” Majidy says, “but is this very young, energetic rising-star researcher.”