Why This Physicist Is Taking Thermodynamics to the Quantum Age

Thermodynamics, developed in the 19th century in the context of the Industrial Revolution, describes the physical concepts of heat, work, and energy (SN: 6/12/24). The field was born out of scientific efforts to understand steam engines. Unlike those clanging, clanging industrial machines, quantum physics describes phenomena on the scale of atoms, electrons, and the like, and has fueled the development of modern technologies such as quantum computers (Serial No.: 28.06.23).

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

Now, however, the two concepts are colliding in quantum engines and other miniature devices. Quantum thermodynamics researchers aim to develop tools to describe heat, work, cooling, and efficiency in quantum systems and to determine the limits of quantum device performance. Yunger Halpern, a physicist at the National Institute of Standards and Technology’s Joint Center for Quantum Information and Computer Science in College Park, Maryland, is leading the effort.

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

One of Yunger Halpern’s most important achievements was to explore what the quantum concept behind Heisenberg’s uncertainty principle might mean for thermodynamics.

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

What if the tea wasn’t a whole cup, but a bundle of just a few atoms? Yunger Halpern wants to know what the exchange would look like. In quantum physics, conserved quantities can be incompatible with each other. That is, they can’t be measured simultaneously. Heisenberg’s uncertainty principle, which states that the better you know the position of a quantum object, the less you know its momentum, and vice versa, gives a famous example (SN: 1/12/22).

“For decades, almost no one really thought about what happens when you have a system and an environment that exchange incompatible quantities,” says Yunger Halpern. It turns out that incompatibility can have real effects on the behavior of the system, she and colleagues note in a study on the subject published in 2023. Nature Reviews Physics. For example, incompatibility can reduce the amount of entropy or disorder that is created in such exchanges. Because the total entropy of an isolated system tends to increase with time, some scientists believe that entropy is closely related to the “arrow of time” that distinguishes the future from the past (SN: 7/10/15). In a sense, says Yunger Halpern, this means that incompatible quantities can make it harder for the system to experience this arrow of time.

Quantum thermodynamics has led to some interesting laboratory demonstrations. For example, a single atom can be turned into a quantum engine that converts heat into work (SN: 14.04.16). Now Yunger Halpern intends to use quantum thermodynamics in practice 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 stimulation by experimenters to operate. Autonomous devices would operate automatically.

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

In a July 2023 arXiv paper, she and her colleagues outline the 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. That means a quantum engine cannot take more energy to drive it than it produces. Quantum physicist Marcus Huber worked with Yunger Halpern to develop these criteria. “I find her brilliant, but also mega-intense and focused,” says Huber of TU Wien in Vienna. “She will bombard you with pertinent and good questions.”

It’s not just her science that’s in the spotlight—her writing, too. Yunger Halpern’s 2022 book, Quantum Steampunk: The Physics of Yesterday’s Tomorrowbrought the field to public attention. She is also a science blogger for the website Quantum Frontiers. Writing, Yunger Halpern says, allows her to explore new ideas without the constraints of scientific publication (fantastical speculations and “crazy” ideas are unlikely to pass peer review). “Thinking really big and wild and as creatively as you feel like on any given day of the month is useful for staying 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, who will soon join Princeton University and recently completed a Ph.D. co-supervised 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 discovered his favorite local ice cream brand—Kawartha Dairy—and sent him a gift card.

Her hobbies are quiet, slow activities: walking, visiting museums. But she weaves an intense passion into her work. “She has very old-fashioned interests and tastes,” says Majidy, “but she is a very young, energetic rising star among researchers.”