A Physicist and Steampunk Enthusiast Explores Thermodynamics in the Quantum World

Snuggling with her parents and brother while watching one of her favorite TV programs, Nicole Yunger Halpern was only 10 when she got her first taste of punk.

She didn’t start listening to the Sex Pistols or sculpt her hair into spikes. But the program she watched, The Adventures of Brisco County, Jr., about a 19th-century bounty hunter who encounters rockets and zeppelins while pursuing a time traveler, exposed her to another kind of punk.

Known as steampunk, this genre of science fiction is set in the Victorian era and blends industrial steam-powered machinery with futuristic technologies like time machines.

Today, Yunger Halpern’s work as an award-winning physicist also straddles two centuries and two worlds – thermodynamics and quantum mechanics.

Developed in the early 1800s, thermodynamics provides the laws governing the flow of energy and its ability to transform from one type, such as heat, to another type, such as mechanical energy. Engineers used thermodynamics during the Industrial Revolution to identify the factors that determine the efficiency of steam engines.

Quantum mechanics, in contrast, describes the physical behavior of the smallest particles in the universe and underlies the operation of 21^st-century devices, such as quantum computers.

Yunger Halpern, a fellow at the Joint Center for Quantum Information and Computer Science (QuICS), a research partnership between NIST and the University of Maryland, calls the combination of these worlds “quantum steampunk.” It’s also the title of her Ph.D. thesis. Science News named Yunger Halpern as one of 10 early- and mid-career scientists to watch for in 2024.

Yunger Halpern’s research has led to some surprising findings that run counter to our everyday experiences.

Mixing Thermodynamics and Quantum Physics

Until recently, many scientists believed the very concept of merging thermodynamics with quantum physics was contradictory, Yunger Halpern says.

That’s because physicists have traditionally applied thermodynamics to huge numbers of particles, like the one thousand billion trillion water molecules in a typical steam engine. In contrast, quantum effects are most apparent in individual particles or very small groupings of atoms or molecules.

The laws of thermodynamics govern how particles exchange heat and other forms of energy with their surroundings and place an upper limit on the amount of energy available to do work, such as pushing a piston.

The laws also dictate that over time, disorder, or entropy, must always stay the same or increase (on average) in an isolated system.

Quantum thermodynamics explores how thermodynamics can be carried over to the tiniest of realms, where randomness and uncertainty hold sway. At the quantum level, pairs of particles can become strongly linked or entangled in a way that non-quantum particles cannot.

The theory may improve the effectiveness of quantum computers and other quantum devices. While still under development, quantum computers have the potential to outperform traditional computers in tasks such as drug design and simulating complex molecules.

Yunger Halpern says learning about quantum thermodynamics may help build better batteries, tiny but efficient quantum engines, and quantum computers.

Eclectic Studies

In studying the intersection of thermodynamics and quantum theory, Yunger Halpern benefits from the interdisciplinary thinking she honed as an undergraduate at Dartmouth College. There, she nurtured a burgeoning interest in physics. She augmented those studies with classes in philosophy, history, and other fields.

“Studying a broad range of subjects enabled me to see what became my subject – quantum thermodynamics – from many different perspectives,” says Yunger Halpern. “It also helped me draw connections between disparate ideas, and that’s probably one of the key stamps of my research nowadays. I see an idea over here in one field and an idea over there in another field, and I connect them.”

In her current work, Yunger Halpern has seized on an intriguing puzzle in the quantum world that most other physicists have overlooked.

According to Heisenberg’s uncertainty principle, certain pairs of properties cannot be measured simultaneously with unlimited precision. Physicists call these incompatible quantities. One such pair is the position and momentum of a quantum particle. At any given moment, the more certain you are about a particle’s position, the less sure you are about its momentum.

In addition, the order in which incompatible quantities are measured makes a difference because the two measurements affect each other. If you measure the position of a particle first and then its momentum, the first measurement influences the second, so that the momentum will be unreliable. Conversely, if you measure the momentum first, the position will become wildly uncertain.

For generations, physicists have assumed that the quantities exchanged in a thermodynamic system, such as the heat and the number of particles it contains, are compatible. In other words, if you measure the amount of heat first, then the number of particles, your measurements can be just as precise as when you measure the number of particles followed by the amount of heat.

That assumption made sense because physicists were studying large non-quantum systems such as steam engines.

But in a quantum system, that may not always hold true. Could incompatibility alter the flow of heat or the transfer of information in a quantum system?

“Over the past 10 years, I’ve been kind of obsessed with asking that question,” Yunger Halpern says. In an article published two years ago in Nature Reviews Physics, she and her colleagues surveyed what may happen when a system and its environment exchange quantities that are incompatible.

Consider the example of adding a blob of red ink to a glass of water. Initially, the blob hasn’t had time to spread out. You can easily distinguish the ink from the clear water. The system is orderly or in a low state of entropy. Over time, as the ink diffuses into the water, the two become thoroughly mixed. It is no longer possible to distinguish the ink from the water. The system is less orderly and has more entropy.

An important part of the concept of entropy production is that it happens over time and generally can’t be reversed easily. We see the ink mixing into the water, but you can’t easily separate the ink and water once they’re mixed. It’s as if nature has picked out an arrow of time – it seems to flow in only one direction, from more orderly to less orderly.

In their review article, Yunger Halpern and her collaborators noted that the exchange of incompatible quantities in a quantum system can decrease the amount of entropy produced in the interaction. The smaller amount of entropy production could mean that some quantum systems don’t experience the arrow of time in the same way as non-quantum systems, Yunger Halpern suggests.

If this all seems abstract, Yunger Halpern is also pursuing the practical. She recently collaborated with experimentalists to create a quantum machine that operates autonomously. That’s in contrast to quantum machines like atomic clocks, which require continual monitoring. Similarly, qubits, the quantum analogs of computer bits, are fragile and so must usually be maintained at ultralow temperatures, often at a high cost and with continual monitoring.

Yunger Halpern and her colleagues recently created an autonomous quantum refrigerator that can cool a quantum bit without the need for constant feedback or monitoring. That’s a key accomplishment because most qubits can only operate if they’re cooled to temperatures just a fraction of a degree above absolute zero.

The finding, which the researchers reported in Nature Physics on Jan. 9, suggests that quantum refrigerators could become as practical as their 19th-century steam-engine counterparts. If quantum computers become more commonly used, device manufacturers will need ways to keep them at the required temperature.

Victorian Devotion

When Yunger Halpern isn’t conducting research, contributing to a blog about quantum physics, collaborating with an artist on a steampunk sculpture, or co-teaching a class on steampunk fiction, she’s exploring all things Victorian.

“I read a lot of books either from the Victorian era or about the Victorian era, for instance, the novels of Anthony Trollope and especially Elizabeth Gaskell,” she says. “Whenever there is a museum exhibition nearby about the Victorian era or the Gilded Age, I feel I have to go because it is my era.”

Papers:

S. Majidy, W.F. Braasch, A. Lasek, T.  Upadhyaya, A. Kalev, and N. Yunger Halpern. Noncommuting conserved charges in quantum thermodynamics and beyond. Nature Reviews Physics, Volume 5, Issue 11, pp. 689-698, 2023. DOI: 10.1038/s42254-023-00641-9.

M.A. Aamir, P.J. Suria, J.A.M. Guzmán, C. Castillo-Moreno, J.M. Epstein, N. Yunger Halpern and S. Gasparinetti. Nature Physics, published online Jan. 9, 2025. DOI: 10.1038/s41567-024-02708-5