Sir Anthony James Leggett
active c. 1938, London, England
British theoretical physicist who explained superfluid helium-3 and pioneered experimental tests of quantum mechanics' limits at the macroscopic scale.
Leggett studied classics before switching to physics, earning his PhD at Oxford in 1964 under Dirk ter Haar. After stints at Urbana, Kyoto, and Cornell, he was professor at Sussex 1971–1982, then moved to the University of Illinois Urbana-Champaign.
In 1973–1975 he decoded the superfluid phases of ³He — the most complex macroscopic quantum system known — showing that its Cooper pairs carry nonzero spin and orbital momentum. He is also known for the 'Leggett–Garg inequality' testing macroscopic realism and the 'Leggett argument' constraining nonlocal hidden-variable theories. He shared the 2003 Nobel Prize.
Nobel-winning work
Superfluid helium-3, discovered in 1972 at Cornell, differs fundamentally from helium-4: ³He atoms are fermions, so superfluidity requires Cooper-type pairing — but the observed A and B phases did not match the symmetric s-wave pairs of BCS. Leggett showed that ³He pairs in a p-wave state (orbital momentum 1) with triplet spin — an 'anisotropic superfluid' of rich internal structure: anisotropy, textures, asymmetric vortices, and connections to axion physics in cosmology.
His theory explained the entire observed NMR phenomenology, and the concept of 'anisotropic pairing' was later used to understand unconventional superconductors and topological quantum liquids. The 2003 Nobel was shared with Abrikosov and Ginzburg for 'pioneering contributions to the theory of superconductors and superfluids'.
Workplaces: University of Illinois Urbana-Champaign, University of Sussex
United Kingdom, United States