CHAMPAIGN, Ill. — Theoretical physicist Sir Anthony James Leggett, widely recognized as a world leader in condensed matter physics and for his pioneering work on superfluidity and the quantum mechanics of macroscopic systems, died March 8. The University of Illinois Urbana-Champaign professor was 87 years old.
Leggett had been the John D. and Catherine T. MacArthur Professor of Physics at the University of Illinois Urbana-Champaign since 1983. His research in theoretical condensed matter physics and the foundations of quantum mechanics won the 2003 Nobel Prize in Physics for his contributions to the theory of superconductors and superfluids.
“Professor Leggett was a brilliant mind, always working on groundbreaking ideas and had such a down-to-earth, humble personality,” said Rashid Bashir, dean of The Grainger College of Engineering and professor of bioengineering at the U. of I. “The world has lost a legend and a wonderful person. He will be dearly missed.”
Leggett was a master of explaining superfluidity, the property of some fluids to flow freely without viscosity. His research extended the legacy of U. of I.’s contribution to the theory of superconductivity ― that some materials lose all electrical resistance when cooled to temperatures close to absolute zero ― developed by researchers John Bardeen, Leon Cooper and J. Robert Schrieffer, for which they received the 1972 Nobel Prize in Physics.
His research was expansive and included considerable theoretical work on the applications of quantum mechanics to collective variables, and on ways to incorporate quantum dissipation, tunneling and coherence into the behavior of macroscopic quantum systems. Leggett’s theories directly led to experiments on macroscopic quantum tunneling and coherence by John Clarke, John Martinis and Michel Devoret, who shared the 2025 Nobel Prize in Physics for this work.
Since the early 1960s, Leggett was interested in the problem of the superfluid liquid helium-3 — a rare isotope of helium that liquefies just above absolute zero. It could not be explained by the then-existing theory, which accounted for the superfluidity of helium-4. Leggett’s theory, developed between 1972 and 1975, provided the explanation for the phenomenon in terms of a new physical mechanism, a novel form of symmetry breaking that he introduced. This was the basis for understanding the superfluid phases of helium-3 as anisotropic pairing of helium-3 atoms.
In 1986, when high-temperature superconductors were discovered, Leggett proposed a test of the symmetry of the new superconductors, leading to an experiment performed at the U. of I. by Dale Van Harlingen, Donald Ginsberg and David Wollman. The 1993 experiment established that new superconductors had “d-wave” symmetry, unlike conventional low-temperature superconductors, which are isotropic.
Leggett was born in Camberwell, South London, in 1938. His mother and father were the first in their families to attend university, where they met and became engaged while students at the University of London.
He began his college education as a classics major, graduating from Oxford University with a B.A. in 1959. After completing his first degree, he began a second undergraduate degree at Oxford, this time in physics. In 1964, after completing his doctorate at Oxford, he joined the U. of I. physics department as a postdoctoral researcher with David Pines and John Bardeen.
“Having Tony as a warm and generous colleague over the years made being in Urbana truly worthwhile,” said Illinois physics professor Gordon Baym, who worked closely with Leggett during his time as a postdoctoral researcher. “He was always way ahead of the rest of us, owing not simply to his being smarter, but also to his boundless energy and working harder. Tony’s enthusiasm for physics and people continued to the end.”
From 1967 until 1983, Leggett was a lecturer at the University of Sussex, England. During that period, he visited Japanese universities on several occasions, as well as the University of Ghana, and then returned to the U. of I. to join the physics faculty until his retirement in 2018.
During his time at the U. of I., he also held a position at the Institute for Quantum Computing in Waterloo, Canada. In 2013, he became the founding director of the Shanghai Center for Complex Physics and in 2023, he became the chief scientist at the Institute for Condensed Matter Theory, a research institute at the U. of I., which in 2023 was renamed the Anthony J. Leggett Institute for Condensed Matter Theory.
Leggett is survived by his wife, Haruko Kinase-Leggett, married in 1973; his daughter, Elizabeth Asako Kinase-Leggett; and his sisters, Judith Leggett and Clare Prangley.
Among his other numerous awards are the Wolf Prize in Physics (2022-23), the Eugene Feenberg Memorial Medal (1999), the Paul Dirac Medal and Prize (1991), the Simon Memorial Prize (1981), the Fritz London Memorial Award (1981), and the James Clerk Maxwell Medal and Prize (1975).
Leggett was a member of the National Academy of Sciences, the American Philosophical Society, the American Academy of Arts and Sciences, the American Philosophical Society and the Irish Literary and Historical Society. He was a Fellow of the Royal Society, the American Physical Society and the American Institute of Physics. He was an Honorary Fellow of the Institute of Physics and a foreign member of the Russian Academy of Sciences. He was knighted by Queen Elizabeth II in 2004 “for services to physics.”
“Tony was a much-beloved colleague. He was so soft-spoken that it was easy to overlook the remarkable extent of his accomplishments,” said Illinois physics department head Vidya Madhavan. “He felt a profound allegiance to both the department and the university. No matter how busy he was, he always made time for colleagues and students, and, despite his many travels, he remained a constant presence in the physics department. Together, he and Haruko opened their home to generations of colleagues, students and postdocs for which we are all very grateful.”
