How Anthony Leggett pushed the boundaries of quantum physics

In my first year of graduate school, I briefly shared an office with a quiet senior graduate student. When we finally talked, I learned that he was “working on the glasses theory with Tony.” Two things became clear: breaking the physics of the glasses was difficult, and I really should have known who Tony was. I met him early enough. The polite septuagenarian from Great Britain spoke with the cadence of a lifelong teacher and an undeniable twinkle in his eye. His full name was Anthony James Leggett: a Nobel laureate, a Knight of the British Empire, a recipient of countless awards, an expert on the ultracold inhabitants of the quantum world, and a theorist who co-created an influential test to determine where that world might end up, a question he had been working on for decades. He died on March 8th, survived not only by his family, but by countless inspired researchers, to whom he was just Tony in his characteristic humble way.

Born in South London in 1938, Leggett attended a Jesuit school where his father taught physics and chemistry before studying classical literature, philosophy and ancient history at Oxford University. But the siren call of physics was louder than ancient texts and dead languages. He earned another degree, this time in physics, and moved to the University of Illinois Urbana-Champaign (UIUC) for postdoctoral training.

At the time, UIUC was rich with physicists studying new types of quantum matter and materials, many of which only revealed their exotic properties when they were made extremely cold. From his past work, Tony was already talking about ultracold physics, but his time at UIUC drew his attention to the problem of a rare form of helium called helium-3. In his Nobel Prize lecturehe recounted how physicists John Bardeen and Leo Kadanoff came to his office to tell him about an ultracold helium experiment that was taking place in the basement. Leggett set out to capture aspects of this experiment using mathematical equations, but got sidetracked. He left the calculation, but would continue to have an on-again-off relationship with ultracold helium-3 for decades to come.

Serendipity stepped in to draw him back to study this strange matter. One day in 1972, he was on vacation when rainy weather thwarted his hiking plans. Instead, he met an experimental friend, Robert Richardson. According to Leggettwhat he heard that day changed his research career forever and led to his Nobel Prize. Richardson described the results of a study of ultracold helium-3, where his team used an imaging method called NMR, which puzzled Leggett so much that once Richardson left, Leggett said he “sat down to try to construct a formal proof that, given the generally accepted laws of quantum and statistical mechanics, the shift observed in the experiments simply could not have occurred.” In other words, he feared that by studying ultracold helium, Richardson and his colleagues might have stumbled upon a crack in quantum physics itself.

Over the course of a few years, Leggett figured out that quantum physics was in fact fine, but ultracold helium-3 was actually unlike any ultracold system that had been studied before. Around this time, the ultracold realm was throwing physicists for a loop. Gases or even some solid materials cool enough and sometimes behave so strangely. For example, at low enough temperature, the electrons in superconductors do not repel as usual, but pair up and carry electricity with perfect efficiency. In other cases, tens or hundreds of thousands of atoms exposed to extreme cold can all occupy the same quantum state, effectively behaving as a single piece of quantum matter rather than distinct individuals. This creates a superfluid, has zero viscosity and can perform unexpected tricks, such as climbing the walls of the container. Was helium-3 a super-something too? Leggett wanted to find out, and he did so rigorously.

He developed a comprehensive theory of ultracold helium-3, a mathematical undertaking that revealed that it was not just one superfluid, but that its atoms could form several different types of superfluid. In describing it, he also discovered a new form of symmetry breaking—a mathematical feature of the new theory of ultracold that could explain previously puzzling measurements from the lab.

Richardson was awarded the Nobel Prize for his experiment with helium-3 in 1966 and Leggett’s Nobel Prize for theory, came in 2003.

“I still remember the euphoria in 2003 the day the Nobel Prize was announced in the early hours of the morning,” he says Smith Vishveshwarwho was my graduate advisor at UIUC. Tony moved to UIUC in 1983, and she began working with him as a postdoctoral researcher in 2002. “He was such a caring, gentle, wise mentor, friend, colleague and inspiration to so many of us.” I can picture him sitting at one of the round tables in the Institute for the Theory of Condensed Matter Physics at UIUC, which now bears his name, lost in thought but never too busy to answer a question.

And Tony was interested in many more questions than just the mystery of superfluid helium-3. It was the glasses study that the older grad student told me about, but Tony was particularly interested in the idea that quantum theory might not work for the whole world, and specifically that it might not work for large objects. Could all the weirdness of quantum physics—how particles are just clouds of possible properties when nobody’s looking—be reduced to tiny objects?

Legget speculated in a 2003 interview after the Nobel Prize ceremony, saying: “If we really still believe. [quantum physics] in the year 3000, then I think in some sense our attitude to the physical world on an everyday level will be radically different from what it is today, because we will have to really face this strangeness, which I believe will be amplified to an everyday level at that time. I think it’s at least as likely, and maybe even more so, that…we’ll find that somewhere along the line, quantum mechanics will break down and some new theory will take over that we currently have no idea about. He said his personal hope was that exactly that would happen.

Searching for the frontier of quantum physics

In search of this elusive quantum decay line, he and Anupam Garg formulated a mathematical test in 1985 that can be used to assess the quantumness of large objects. You can observe the behavior of an object at different times, plug those observations into an equation now called the “Leggett-Garg inequality,” and tell whether the rules of quantum physics still hold or not. In recent years, Leggett-Garg’s experiments have been performed on several systems, from particles of light to tiny crystals, and researchers are constantly pushing them to ever larger scales.

Leggett’s questions about the relationship between the macroscopic world and quantum physics also planted experiments that were awarded a Nobel Prize just last year. “I heard him talk about it in the early 1980s, and so did others. We took his proposal and turned it into a very good experiment,” says John Martinis of the quantum computing firm QoLab, which won the Nobel Prize for showing that quantum effects can occur on scales as large as circuits made of layers of superconductors and insulators. Leggett already had a deep understanding of how such circuits could test the existence of macroscopic quantumness, which was a big motivation for Martinis and his team to carefully assemble them in the lab, he says.

“I think it’s fair to say that Tony was able to look at what everyone else thought was a minor blip on the chart and recognize it as a signal of something completely new. he wrote his former student David Waxman at Fudan University in China. “Tony was extremely 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 about how long it will take, and don’t be discouraged by the assurances of your fellow physicists that the questions are well understood.” once advised. He then added that “no piece of honestly conducted research goes to waste,” even if it ends up sitting in a drawer for decades before it sparks a new idea.

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

topics:

• quantum mechanics/
• quantum physics