Theoretical physicist Anthony J. Leggett, renowned for his seminal work on superfluidity and quantum science, might not have become a scientist at all. As an academically inclined student in mid-20th-century Britain, he was encouraged to study classical Greek and Latin. His first degree, from the University of Oxford, was in Literae Humaniores, a traditional program of study combining ancient literature and modern philosophy.

He considered pursuing a doctorate in philosophy and an academic career in the subject, but he was frustrated by the absence of objective standards for determining the quality of one’s work. Instead, he found himself drawn to physics. He would later quip that, unlike philosophy, physics allows one to be “wrong for interesting and nontrivial reasons.” So, after completing his first bachelor’s degree, he was admitted to pursue a second at Oxford in physics. 

He went on to have an exceptional career, making important contributions to condensed matter physics and the foundations of quantum mechanics. He was a co-recipient of the 2003 Nobel Prize in physics for developing new theoretical tools to describe superfluid helium-3. His work on macroscopic quantum tunneling inspired the experiments that were awarded the 2025 Nobel Prize in physics. It also provided the theoretical basis for superconducting qubits, a major technology used in today’s quantum computers.

Beginning in 1983, he held the John D. and Catherine T. MacArthur Chair in The Grainger College of Engineering’s Department of Physics, and the department’s Anthony J. Leggett Institute for Condensed Matter Theory now bears his name. He was adored by students and colleagues for his kind, gentle demeanor. He worked to promote the warm, collegial atmosphere that makes Illinois an epicenter of condensed matter physics.

Despite his success as a physicist, he was adamant that he never regretted his early studies of classics and philosophy. For Leggett, physics was not above questions that some might dismiss as philosophical rather than scientific. He pushed accepted theories to their extremes to see if they would fail. He worked in both condensed matter physics and quantum science to understand nature as it really is. 

Leggett’s former student Matthew Fisher, now a professor at the University of California, Santa Barbara, said, “Tony’s curiosity ran deeper than any single physical system. At a time when most physicists treated quantum mechanics as a calculational tool, he insisted on asking whether it was actually true: that superposition on macroscopic scales was a question of experiment, not philosophy. He showed me that even seemingly philosophical questions about nature need not be beyond scientific rigor, requiring only patience to find the right experimental handle, and courage to attack them straight on.”

Leggett passed away on March 8, 2026, at the age of 87. The Grainger College of Engineering mourns the loss of one of the greatest scientific minds of our time, but it also celebrates his singular achievements and the legacy he leaves behind.

Superfluidity in liquid helium-3

By training, Leggett was a specialist in condensed matter physics, studying how large collections of atoms and molecules combine to form macroscopic systems. It was work in this realm that led to his Nobel Prize. Specifically, he developed the theory explaining superfluidity in liquid helium-3. In doing so, he demonstrated an unexpected connection with superconductivity.

The modern understanding of superfluidity and superconductivity is that they are two instances of a low-temperature phenomenon called macroscopic occupation. In both cases, the substance becomes so cold that the constituent particles condense into the lowest energy state allowed by quantum mechanics. This causes superfluids to flow without friction and superconductors to conduct electricity without resistance.

Superfluidity occurs in both stable isotopes of helium. Unlike the more common helium-4 atoms, helium-3 atoms have difficulty condensing to form a superfluid phase. Their quantum properties make them naturally resistant to low-energy condensation. In this regard, helium-3 atoms have more in common with electrons in a metal than they do with helium-4 atoms.

The superfluid phase of helium-3 had not yet been observed when the Bardeen-Cooper-Schriefer (BCS) theory was introduced in 1957. It provided the explanation that metallic electrons overcome their resistance to condensation by first pairing off to form composite units. When these composite electron units, known as Cooper pairs, condense into their quantum ground state, the metal becomes superconducting. In the following years, many physicists conjectured that Cooper pairing would need to occur before helium-3 could achieve superfluidity.

Photo Credit: Darrell Hoemann / The News-Gazette

After superfluid helium-3 was finally observed in 1972, Leggett spent almost a year developing a comprehensive theory of the phenomenon. He published a landmark paper in 1973 that identified the Cooper pairing mechanism, worked out the structure of the pairs, and explained the strange magnetic features that the data indicated. Subsequent experiments throughout the 1970s confirmed all his predictions.

In demonstrating that the same fundamental mechanism can underpin both superfluidity and superconductivity, Leggett advanced our understanding of low-temperature physics by a giant leap. Lessons from his theory permeate modern research in condensed matter physics and particle physics, and he took pride in the fact that superfluid helium-3 is one of the most complex physical systems for which an exact mathematical description is available. 

Macroscopic quantum tunneling

Besides condensed matter theory, Leggett was also interested in the conceptual foundations of quantum physics. He devoted much of his research to the question of macroscopic quantum behavior: whether objects familiar to us on human scales can be made to display the behaviors of molecules, atoms and particles described by quantum theory. He used his condensed matter background to propose an experiment that would reveal exactly such behaviors.

In a 1978 paper, Leggett suggested that superconductors are strong candidates for realizing macroscopic quantum behavior, specifically quantum tunneling, wherein an object overcomes an apparently insurmountable energy barrier. The phenomenon is central to radioactive decay and semiconductor device design. It is also the basis for experimental superconducting quantum interference devices, or SQUIDs, in which superconducting electrons tunnel through a barrier.

Leggett reasoned that because SQUIDs operate at very low temperatures and experience very low electrical resistance, the electrons must interact very weakly with the surrounding environment. Since environmental noise and dissipation are what usually destroy quantum properties, it should be possible for all the superconducting electrons in a SQUID to cohere together and tunnel as a single large-scale quantum entity.

The main difficulty was working out how small, but non-negligible dissipation impacts macroscopic tunneling behavior. In 1981, Leggett and his graduate student Amir Caldeira published a theory for how a macroscopic quantum entity tunnels when dissipation is present. 

The Caldeira-Leggett theory, as it came to be known, was subsequently verified in the 1984-1985 experiments of John Clarke, Michel Devoret and John Martinis. The three demonstrated that a real SQUID device could enter a macroscopic quantum state, in which all electrons behave as a single quantum unit, and tunnel into new states inaccessible by non-quantum means. The 2025 Nobel Prize in physics was awarded for this experiment, but the Nobel committee explicitly acknowledged Leggett’s contributions in predicting and developing a theory for the behavior.

When quantum computing research began in the 1990s, Leggett’s theoretical work indicated that superconducting devices were very strong candidates for quantum processing units, or qubits. Today, many academic researchers, in addition to companies such as Google, IBM and Rigetti, are actively developing quantum computers with superconducting qubits thanks to Leggett’s work.

Quantum foundations

Leggett’s work on macroscopic quantum tunneling was just one part of his longstanding drive to push quantum physics to its limits. 

Quantum theory, although effective at explaining the microscopic world of molecules, atoms and particles, makes claims that appear to contradict the understanding of reality we derive from our macroscopic experiences. Since its introduction, questions have been raised about the extent to which quantum theory is a literal description of reality or should be understood as only a useful tool for calculations. Working scientists were long urged to set aside such broad questions, which today are known as the discipline of quantum foundations, and focus on what could be experimentally tested.

Leggett became interested in quantum foundations in the late 1960s, when it was still regarded as a fringe field. He leveraged his condensed matter background to look for ways to probe quantum reality. Initially, he believed that the strange data found for superfluid helium-3 indicated that quantum theory had failed. Although his final model was based on standard quantum theory, he continued to push the theory’s limits with his investigation of macroscopic quantum tunneling.

In 1985, Leggett and his graduate student Anupam Garg published a mathematical criterion for probing the quantum phenomenon of superposition. One of quantum theory’s seemingly strangest predictions, the idea of superposition is that physical objects can exist in multiple states at the same time until they are measured. The criterion is a mathematical inequality wherein measurements of a system that exists in exactly one state at time must produce a numerical value below a certain threshold. 

The Leggett-Garg inequality, as it is now known, has been shown to be violated in systems ranging from superconductors to neutrinos, meaning that the measured value exceeds the threshold for possibly existing in a single state at a time. The conclusion is that superposition is real and objects do in fact occupy multiple states at once, affirming that quantum physics accurately describes the true nature of the systems it models.