# New Faculty Profile: Thomas Barthel

*Wednesday, July 30, 2014*

In July 2015, **Thomas Barthel** will join the Duke faculty as the Charles H. Townes Assistant Professor of Physics. Barthel studies the quantum mechanics of many-particle systems using computer simulations and analytical techniques.

“I like to solve problems which either have high technological interest or are of some fundamental nature,” Barthel said. “I want to contribute to understanding these very complex problems.” As one example, he cited high-temperature superconductivity, which still lacks a thorough theoretical explanation though it was first demonstrated experimentally several decades ago.

A native of Leipzig, Germany, Barthel studied in Heidelberg and Aachen, and received his doctoral degree from RWTH Aachen University. He is currently a research associate at the Laboratoire de Physique Théorique et Modéles Statistiques, jointly run by University Paris-Sud and CNRS (France’s National Center for Scientific Research).

Barthel develops and employs methods that model the behavior of many-particle systems on an atomic scale—behavior which is governed by quantum mechanics rather than the familiar laws of classical mechanics that apply to balls, cars and other everyday objects.

“Quantum theory is essential for understanding fundamental properties like the stability of matter, surprising effects like the frictionless flow of Helium-4 below 2 Kelvin, or advanced technological applications like the lasers and masers that Charles H. Townes worked on,” Barthel said. Charles H. Townes, a Nobel laureate for whom the endowed professorship is named, earned a master’s degree in physics at Duke in 1937.

Barthel studies ultra-cold gases and solids. The number of interacting particles in these systems can range from 100 to 10^{23}, adding complexity to a situation that’s already pretty complex. In the context of quantum mechanics, the state of the system can be a superposition, or combination, of all of its possible classical states. “As a consequence, the number of degrees of freedom grows exponentially with the system size instead of linearly as in the corresponding classical system,” he said.

With such a large number of particles and degrees of freedom, the systems typically can’t be described exactly. “The key is to try and find suitable approximations and identify a manageable reduced set of relevant effective degrees of freedom,” he said. To meet this challenge, Barthel is developing novel numerical techniques to approximate the many-particle systems. “These techniques make it possible to go far beyond what we can do with pen and paper,” he said.

After years of working on quantum mechanics he says he’s developing an intuition about how these systems behave. “That’s what I like,” he said. “Trying to get an intuition for these many-particle systems. I want to get a feeling for this totally different world. The longer you work on this, the more intuition you will have.”

However, quantum mechanics still offers plenty of surprises. Barthel quoted Richard Feynman, a famous physicist who once said, “I think I can safely say that no one understands quantum mechanics.”

The very complexity that makes understanding these systems difficult also offers the potential for technological advances, such as quantum computers, which would be able to process much more information much more quickly and securely than today’s digital computers.

One roadblock to quantum computing is the issue of decoherence, which is the loss of quantum information due to the “coupling” of a quantum system to its environment. Barthel and a colleague recently demonstrated that many-particle systems can behave differently than systems with fewer particles in terms of how they move toward decoherence. “One can tune interactions in the quantum system such that it moves toward decoherence much more slowly,” he said.

In his free time, Barthel enjoys hiking, reading history and philosophy, and playing basketball. He expects to add “watching basketball” to that list once he gets to Durham. “I’ve heard Duke has one of the best teams and a big coach who also coaches the Olympic team, so I’m looking forward to following some games,” he said. “In the United States, basketball is much more important than in Europe. It will be interesting to see how the discussion works.”

Barthel is also looking forward to collaborating with students and other scientists at Duke, not only on condensed matter, but also on quantum chemistry, which involves smaller systems of electrons in atoms or molecules.

Potential collaborators include physicists **Harold Baranger**, **Albert Chang**, and **Gleb Finkelstein**, and mathematician **Jianfeng Lu**. Barthel enjoys working not just with other theoreticians, but with experimentalists as well. “I can give them some ideas for interesting experiments,” he said, “or if they have done a nice experiment, I can help explain the results.”

*Mary-Russell Roberson is a freelance science writer who lives in Durham.*