Anthony J. Leggett Memorial and Conference 2026 - Tony Leggett
Tony Leggett
In this Conference and Memorial, we celebrate the magnificent scientific life of our beloved colleague Sir Anthony J. Leggett. Known for his depth of intellect and expansive view of physics, Tony, as he was known to us, was also deeply human. He insisted on being called simply Tony, a true sign of his inherent humility, a natural counterweight to his prodigious physics. Tony was born in London, UK, on 26 March1938 and passed away in Urbana, IL, on 8 March 2025.
Tony rose to prominence in theoretical physics by unraveling the Cornell experiments on superfluidity in helium-3. Lee, Osheroff and Richardson used nuclear magnetic resonance (NMR) and thermodynamic measurements to find three distinct phase transitions as a function of temperature and magnetic field below 2.5 millikelvin. In fact, Tony pursued this problem because he thought surely such a radically different phenomenon must lie outside the purview of quantum mechanics. As it would turn out, some form of superconductivity and the foundations of quantum mechanics were the two foci of Tony’s scientific career, though this was not the intent. In 1973, he showed that the macroscopic quantum coherence of the superfluid state was enhanced a tiny dipolar interaction which allowed him to explain the NMR results. More importantly his work allowed him to identify one of the discovered phases as the `A phase’ as the Anderson-Brinkman-Morel proposed state while the other the `B phase’ as the Balian-Werthamer state. Leggett’s work led to the full characterization of the superfluid states of liquid helium-3. This work led to Tony’s 2003 Nobel Prize, which he shared with Alexey Abrikosov and Vitaly Ginzburg, for pioneering contributions to the theory of superconductors and superfluids. A natural spinoff of Tony’s work in helium-3 is the current field of anisotropic superconductors.
Photo by Rick Kubetz
However, the larger question Tony’s work helped delineate is the essential tension between quantum coherence and interactions with an environment that could ultimately lead to dephasing. Precisely how does a physical system retain its quantum nature in the presence of an environment is at the heart of his 1983 paper by Tony and his then graduate student Amir Caldeira. They considered a `two-level system’ in the form of a particle in an energy landscape with two minima coupled to an external environment. Without the coupling to the environment, the particle is in a superposed quantum states in which it is at both minima at the same time. However, when the coupling is turned on, quantum coherence is eventually lost, and the particle is localized at either minimum as one would expect in classical physics. Another problem they considered is the physics of a superconducting quantum interference device (SQUID). In an isolated superconducting ring, a magnetic field can induce a current which in the absence of dissipation will stay forever without decay. However, if somewhere on the ring there is a defect creating a Josephson junction, the SQUID will be able to tunnel between a state with no current to a state with current. This scheme is the basis of current superconducting qubits used in all present-day quantum computers. The Caldeira-Leggett theory describes the fate of quantum coherence in such a device, and it is the engine that drives quantum computation. This theory was brilliantly confirmed by the experiments by Michel Devoret, John Martinis and John Clarke who won the 2025 Physics Nobel Prize for this work whose experiments were entirely motivated by the Caldeira-Leggett work.
Anthony Leggett and his daughter.
After completion of his PhD at Oxford University, he came to the Physics Department of the University of Illinois in 1964-1965 as a postdoc with John Bardeen and David Pines. In 1967 he joined the faculty of the University of Sussex where he stayed until 1983. In 1983 he returned to the Department of Physics of the University of Illinois as the MacArthur Professor of Physics. After his retirement in 2018 he became the MacArthur Emeritus Professor of Physics. In 2023 the Institute for Condensed Matter Theory of the University of Illinois became The Anthony J. Leggett Institute for Condensed Matter Theory.
Tony was a true Ambassador of Physics having visited, sometimes for long stays multiple countries particularly Japan, Canada, China and India as well as Spain, Iran and Ghana among many other countries.
Despite his multiple honors, including being knighted as Sir Anthony J Leggett in 2004, Tony was a very modest and unassuming man. He always interested in discussing new problems and discoveries. He was always ready to discuss with students, postdocs and visitors. He patiently listened to points of view different from his. At most he would express his disagreement just by a slight motion of his head. In our Physics Department he will always be remembered as a good citizen always ready to help. He was also well known in our community as a regular rider of the Red Bus line and for his participation in the Physics booth at the Urbana Framers Market on Saturday mornings.
Professor Eduardo Fradkin
Director of the Anthony J. Leggett Institute for Condensed Matter Theory,