How Anthony Leggett Revolutionized Quantum Physics: Breaking New Boundaries

During my first year of graduate studies, I shared an office with an older graduate student who was quietly conducting pivotal research. Upon conversing with him, I discovered he was “working with Tony on the theory of glasses.” It soon became evident to me that the physics behind glasses posed significant complexities and that I should have recognized Tony’s name sooner. My initial meeting with Anthony James Leggett was enlightening—a courteous British gentleman in his 70s, with the wisdom of a seasoned educator and an undeniable sparkle in his eye. He was a Nobel laureate, knighted by the British Empire, recipient of numerous accolades, and a pioneer in quantum theory, notably examining the enigmas of cold quantum realms. He passed away on March 8, leaving behind a legacy fueled by his integrity, curiosity, and numerous aspiring scientists, yet to many, he simply remained Tony.

Born in 1938 in South London, Leggett attended a Jesuit school where his father instructed in physics and chemistry. Originally earning a degree in classical literature, philosophy, and ancient history from Oxford University, he ultimately succumbed to the allure of physics, pursuing it further at the University of Illinois at Urbana-Champaign (UIUC) for his doctorate.

At that time, UIUC served as a hub for physicists delving into novel quantum materials. Many of these materials exhibited extraordinary characteristics only at ultra-low temperatures. Leveraging his prior expertise in cryogenics, Tony redirected his focus towards the peculiarities of helium-3. He recounted a memorable encounter with physicists John Bardeen and Leo Kadanoff, who introduced him to their groundbreaking experiments with ultracold helium. Although he attempted to encapsulate these discoveries mathematically, initial distractions led him to maintain an intricate relationship with helium-3 over the next decade.

In a serendipitous twist during a rain-soaked 1972 vacation, he met experimentalist Robert Richardson, whose discussion of helium-3 experiments significantly impacted Leggett’s research career. Following their conversation, Leggett aimed to develop a formal proof demonstrating the impossibility of observed phenomena aligning with established quantum mechanics. This moment hinted at potential discrepancies within the framework of quantum physics itself.

Leggett’s subsequent investigations revealed that while quantum principles held, helium-3 exhibited unprecedented traits rarely seen in other cryogenic systems. As researchers explored the unusual behavior of materials under extreme cold, they uncovered effects like superconductivity, where electrons cohesively pair in a unique quantum state—enabling perfect electrical conductivity. Intrigued by whether helium-3 could exhibit comparable superfluid qualities, Leggett meticulously delved into its properties.

Ultimately, Leggett crafted a comprehensive theory around ultracold helium-3, establishing that its atoms can form multiple types of superfluids and introducing a novel form of symmetry breaking, elucidating previously obscure experimental results.

Richardson had won the Nobel Prize for his 1966 helium-3 research, while Leggett received his Nobel Prize for groundbreaking theoretical contributions in 2003.

Reflecting on the announcement of his Nobel Prize in 2003, Leggett expressed the elation felt by many during that early morning news. His former graduate advisor, Smitha Vishveshwara, attested to his profound kindness and wisdom, which inspired countless individuals at UIUC. Tony joined the university in 1983, and I had the privilege of working with him as a postdoctoral fellow starting in 2002. He was often deep in thought, too busy at his roundtable in the Institute for Condensed Matter Physics, now bearing his name, to engage with anyone.

Beyond his groundbreaking work on superfluid helium-3, Leggett was passionate about broader questions that questioned the foundations of quantum physics. He delved into intriguing theories regarding whether the quantum realm might apply to large-scale objects—a notion he explored in an interview post-Nobel Prize celebration. Leggett noted, “If we genuinely adhere to quantum theories, I believe the perceptions we hold about the physical world will differ significantly by AD 3000.” He intriguingly speculated about a potential evolution in physical understanding, pondering new paradigms that may emerge.

Exploring Quantum Physics Frontiers

To probe the fascinating boundaries of quantum mechanics, Leggett, alongside Anupam Garg, developed a mathematical test in 1985 for assessing the quantum characteristics of large objects. This experiment, now known as the Leggett-Garg inequality, evaluates object behavior over time—offering insights into whether quantum laws govern these entities. Researchers worldwide have since executed the Leggett-Garg experiment on various systems, including photons and minuscule crystals—sparking advancements in quantum physics.

His inquiries regarding the intersection of macroscopic occurrences and quantum phenomena laid the groundwork for another Nobel Prize-winning experiment last year. John Martinis, from the quantum computing company QoLab, highlighted that collaboration on a large-scale circuit experiment stemmed from ideas Leggett initially discussed in the early ’80s. The work confirmed the manifestation of quantum effects in systems of superconducting circuits, echoing Leggett’s extensive knowledge that inspired Martinis and his team as they approached lab construction.

Underlining Leggett’s keen observational talents, David Waxman, a former student, noted, “Tony had an exceptional ability to perceive what others might overlook—he saw potential where many dismissed a mere fluctuation on a graph as trivial.”

Leggett consistently advised young physicists to advocate for their inquiries. He remarked, “If conventional wisdom mystifies you, take time to unravel it, and don’t succumb to peer pressure asserting that it is well understood.” He emphasized that “research conducted with integrity is never fruitless,” allowing for new perspectives to emerge from long-abandoned ideas.

Although I departed UIUC in spring 2020, I can still envision him—an intellectual giant—engaged in profound contemplation at his desk. I firmly believe he never ceased his quest for knowledge, perpetually inclined to uncover nature’s hidden secrets. I wish I had explored the unexplored research awaiting revelation within his desk drawers.