How Anthony Leggett pushed the boundaries of quantum physics

During my first year in graduate school, I shared an office with an older student who was quietly working on the theory of glasses with someone named Tony. It quickly became evident that understanding the physics of glasses was a complex task, and I realized I should have known who Tony was. I soon met him—a polite British man in his 70s, known as Anthony James Leggett, a Nobel laureate, knight of the British Empire, and a leading figure in quantum physics. He was a theorist who co-developed a significant test to explore the boundaries of the quantum world, a pursuit he followed for decades. Leggett passed away on 8 March, leaving behind not only his family but many researchers inspired by his humble character.

Born in South London in 1938, Leggett attended a Jesuit school where his father taught physics and chemistry. Initially, he pursued a degree in classical literature, philosophy, and ancient history at the University of Oxford. However, his passion for physics eventually led him to obtain another degree in the subject. He then moved to the University of Illinois Urbana-Champaign (UIUC) for postdoctoral training.

UIUC was bustling with physicists exploring new quantum materials, many of which showed unique properties at extremely low temperatures. Tony was already knowledgeable about the physics of the ultracold, but his time at UIUC introduced him to the challenges posed by helium-3. As recounted in his Nobel lecture, physicists John Bardeen and Leo Kadanoff informed him about an ultracold helium experiment in the basement. Leggett attempted to model this experiment mathematically but was sidetracked, though he remained engaged with ultracold helium-3 research for a decade.

A chance encounter in 1972, while on a rainy vacation, reconnected him with this field. He met Robert Richardson, an experimentalist friend, who explained a study on ultracold helium-3 using NMR imaging. The results were so perplexing to Leggett that he immediately tried to prove that the observed shifts were impossible under known quantum and statistical mechanics laws. He feared they might have uncovered a flaw in quantum physics.

Eventually, Leggett demonstrated that quantum physics remained intact, but helium-3 exhibited unprecedented ultracold properties. At that time, physicists were already puzzled by the strange behaviors of gases and solids at low temperatures. Electrons in superconductors, for instance, pair up instead of repelling each other, enabling perfect electrical conduction. Similarly, atoms exposed to extreme cold can enter the same quantum state, forming superfluids that can climb container walls. Leggett rigorously investigated whether helium-3 was also a superfluid.

He developed a thorough theory of ultracold helium-3, revealing it could form multiple types of superfluid. In doing so, he discovered a new form of symmetry breaking—a mathematical aspect explaining the previously puzzling experiments.

Richardson had received the Nobel prize for his helium-3 experiment in 1966, and Leggett was awarded the Nobel in 2003 for his theoretical contributions.

“I still remember the communal euphoria in 2003 on the day the Nobel prize was announced in the wee hours of the morning,” says Smitha Vishveshwara, who was my graduate advisor at UIUC. Tony moved to UIUC in 1983, and she came to work with him as a postdoctoral researcher in 2002. “He was such a caring, gentle, wise mentor, friend, colleague and inspiration for so many of us.” I can picture him sitting at one of the round tables in the institute for condensed matter physics theory at UIUC, which now bears his name, engrossed in thought but never too busy to answer a question.

And Tony was interested in so many more questions than just the mystery of superfluid helium-3. There was the study of glasses that that older graduate student told me about, but Tony was especially gripped by the idea that quantum theory may not apply to the whole world, and specifically that it may not work for large objects. Could all the weirdness of quantum physics – like a particle being mere clouds of possible properties when no one is looking at it – be restricted to tiny objects only?

Legget speculated about this in a 2003 interview following the Nobel prize ceremony, saying: “If we really do still believe [quantum physics] in the year 3000, then I think in some sense our attitude towards the physical world at the everyday level will be radically different from what it is today, because we will really have had to face up to this weirdness, which by that time I’m confident will have been amplified to the everyday level. I think it’s at least equally probable and perhaps more so, that…we will find that somewhere along the line quantum mechanics breaks down and some new theory, of which we can have at present no conception, will take over.” He said his personal hope was that exactly this would happen.

Finding the edge of quantum physics

Seeking to find the boundary where quantum mechanics might fail, Leggett and Anupam Garg developed a mathematical test in 1985 to evaluate the quantum nature of large objects. By observing an object over time and applying the “Leggett-Garg inequality,” researchers could determine if quantum physics rules apply. This test has been used on various systems, from light particles to small crystals, with experiments continuously being scaled up.

Leggett’s exploration of how quantum phenomena might manifest in the macroscopic world inspired experiments recognized with a Nobel prize recently. “I heard him talk about this in the early ‘80s, and others did too. We took his proposal and turned it into a very good experiment,” says John Martinis at the quantum computing firm QoLab, who was recognized for showing that quantum effects can appear in large-scale systems, such as circuits made of superconductors and insulators. Leggett’s insights into these circuits motivated Martinis and his team to construct them meticulously in the lab.

“I think it is fair to say that Tony could look at what everyone else dismissed as a minor glitch on a graph and recognise it as signalling something completely new,” wrote his former student David Waxman at Fudan University in China. “Tony was extraordinarily sensitive to what nature was trying to say.”

Leggett’s own advice to young physicists encouraged the same approach. “If there’s something in the conventional wisdom that you don’t understand, worry away at it for as long as it takes and don’t be deterred by the assurances of your fellow physicists that these questions are well understood,” he once advised. Then, he added that “no piece of honestly conducted research is ever wasted”, even if it ends up sitting in a drawer for decades before spurring some new idea.

I left UIUC in the spring of 2020, and even at that time you could still catch a glimpse of Tony in his office, working into his 80s. I truly believe that he never stopped listening to nature with that famous curiosity and care. I wish I could have looked at whatever studies were still waiting for their moment in his desk drawers.