Anthony J. “Tony” Leggett

1938–2026

Reprinted from The Proceedings of the National Academy of Sciences (PNAS) under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND). Copyright the authors. Originally published in PNAS on September 23, 2026.  https://www.pnas.org/doi/10.1073/pnas.2630503123

Anthony James Leggett: Explorer of quantum mechanics,
from fluids and solids to the macroscopic world

by Gordon Baym and Eduardo Fradkin

Tony Leggett (1938–2026) was one of the great theoretical physicists of his generation, specializing in the quantum mechanics of fluids and solids. Rightly recognized by the 2003 Nobel Prize in Physics for his explanation of the experiments discovering the superfluidity of helium-3, he will be remembered equally for his pioneering inquiries into the transition from the quantum to the classical world and the extent to which we can observe quantum phenomena on macroscopic scales.

Anthony James Leggett died on March 8 of this year, just shy of his 88th birthday. Tony, as he was known, was a giant of theoretical physics. His contributions to shaping our understanding of quantum liquids and solids rank with those of L.D. Landau, P.-G. de Gennes, and P.W. Anderson, the three outstanding theorists in this field in the second half of the Twentieth Century. Tony was awarded the Nobel Prize in Physics in 2003 (together with A. Abrikosov and V. Ginzburg) for the central role he played in explaining the experiments that discovered the superfluidity of liquid helium-3 in 1972. For the discovery, experimentalists David Lee, Douglas Osheroff, and Robert Richardson of Cornell were awarded the Nobel Prize in Physics in 1996. Tony was also the leading theorist in figuring out the applicability of quantum mechanics in the macroscopic world, thus shaping our understanding of dissipation and decoherence of large quantum systems.

Tony was born in London in 1938, and although his father taught physics, he went to Balliol College in Oxford to study the mix of philosophy and classics called “Greats.” But with the Soviet Sputnik moment in 1957, he went on to study physics, earning his PhD with Dirk ter Haar in 1964 at Oxford. Tony then came to the University of Illinois as a postdoc with David Pines and John Bardeen in 1964, where he laid out the fundamentals of superconductivity in the presence of strong particle interactions, work that combined with his deep understanding of magnetic resonance led to his pioneering work on superfluid helium-3. Tony returned to the United Kingdom in 1967 as a lecturer at the University of Sussex and eventually became full Professor, prior to returning to the University of Illinois in 1983 as the MacArthur Professor of Physics. Among his many other honors, he received the 2003 Wolf Prize in Physics, the 1981 Fritz London Memorial Prize in low temperature physics, was knighted by Queen Elizabeth II in 2004, and was elected to the Royal Society in 1980, the American Philosophical Society in 1991, and the National Academy of Sciences in 1997.

His Science

Tony learned about the discovery of helium-3 superfluidity when Richardson came to visit him in Sussex to tell him about the Cornell experiments and ask for his help in their interpretation. Since the work of P. Kapitsa in 1937, it was known that the common helium isotope, helium 4, became superfluid below 2.17 K. This phenomenon was soon explained as the Bose–Einstein condensation of helium-4 atoms, which are bosons. But helium-3 atoms are fermions, obeying the exclusion principle, and cannot condense in this way. Instead, they behave similarly to electrons in a metal. Since electrons can become superconducting, the possibility of helium-3, itself electrically neutral, becoming superfluid was an enticing possibility. Arguing in analogy with the Bardeen–Cooper–Schrieffer theory of superconductivity, two groups of theorists in the 1960s, Anderson, William Brink man, and Pierre Morel (ABM) and Roger Balian and Richard Werthamer (BW), had proposed two possible superfluid states for liquid helium-3, but it remained unclear how much one had to cool the fluid to make it superfluid in either state. Lee, Osheroff, and Richardson, searching for such superfluidity using NMR and thermodynamic measurements, succeeded in finding three distinct phase transitions in helium-3 at temperatures below 2.5 millikelvin, and varying applied magnetic fields.

Interpreting the perplexing NMR results, Tony made his first key contribution. He showed that the macroscopic coherence of the state enhanced a tiny dipolar interaction, leading to the NMR signals. More importantly, he identified one of the discovered phases, the “A phase,” as the ABM state and the other, the “B phase,” as the BW state. His work led to the full characterization of the superfluid states of liquid helium-3 and was the basis for his Nobel Prize.

Tony also had a lifelong interest in understanding how microscopic physical systems, governed by the probabilistic laws of quantum mechanics, behave when they interact with the environment, governed by deterministic classical physics. A prototypical quantum effect is tunneling. In classical physics, a particle trapped in a region by energy barriers will remain trapped forever. However, in quantum mechanics, the particle always has some probability of escaping. An example of this is the radioactive decay of the nucleus of a heavy atom by the emission of an “alpha particle” (a helium-4 nucleus). What happens, Tony asked, when such a system is not isolated but interacts with the environment? His view was to regard the interaction as constantly “measuring” a property of the particle, such as its location—in effect a quantum mechanical version of Brownian motion. In the early 1980s, Tony and his then graduate student Amir Caldeira coauthored three groundbreaking papers in which they investigated the problem of macroscopic quantum tunneling and coherence. To illustrate, they considered a particle in an energy landscape with two equally deep minima coupled to an external environment. In the absence of coupling, the particle is in a quantum superposition of being at both minima, but when the coupling is turned on, quantum coherence is eventually lost and the particle is localized in one of the two minima as one would expect in classical physics. This work explained the conditions under which quantum tunneling and coherence can be observed in macroscopic systems.

Caldeira and Leggett immediately applied their work to the physics of a superconducting quantum interference device (SQUID), a ring of a metal in a superconducting state. In a pristine isolated ring, a magnetic field can induce a current which in the absence of dissipation will stay forever without decay. But if somewhere on the ring there is a defect creating what is known as a Josephson junction, the SQUID in principle will be able to tunnel between a state with no current to a state with current. The Caldeira–Leggett theory describes the fate of quantum coherence in such a device and underlies the superconductor technology of current quantum computers. This theory was brilliantly confirmed by the experiments by Michel Devoret, John Martinis, and John Clarke, for which they won the 2025 Physics Nobel Prize. These experiments were entirely motivated by Tony’s work.

Outside of Physics

Tony hardly confined his time to the blackboard. Fluent in some six languages, he traveled abroad extensively and had a worldwide influence on physics. While a postdoc in Kyoto in the mid-sixties he wrote the highly influential, “Notes on the writing of Scientific English for Japanese Physicists,” describing, in analogy with trees, that while the structure of Japanese science writing traditionally first describes the leaves, and works down to the trunk, western scientific writing begins with the trunk and then branches out eventually to the leaves. He taught extensively in Africa, including Ghana and South Africa, developing close lifelong collaborations with physicists from countries such as Sudan and Jordan, and was founding director of the Shanghai Center for Complex Physics of Jiao Tong University. Tony also loved mountains, which he ran through with the spirit and skill of a mountain goat. One could always turn to Tony for wise and thoughtful discussions in all areas of physics. Kind and generous, he and his wife of half a century, Haruko, always opened their home to friends, visitors, and students old and new. Tony Leggett was an extraordinary colleague and friend.