Nobel Prize "for his pioneering contributions to the theory of superfluids".
These are unique materials as they present quantum characteristics at the macroscopic scale (in other systems usually these properties manifest themselves only at the atomic scale). The mechanism by which these unusual properties are acquired is that a large number of the particles forming the material enter in a cooperative state, in which they share a common quantum wave function, extended over macroscopic lengths. This produces, for example, the zero viscosity of superfluids, or the zero electrical resistance of superconductors, in both cases because of the impossibility of the collective wavefunction to experience changes over continuous values (quantization). These properties are important not only for their potential application in the fields of materials technology and science, but also because they provide a unique opportunity to study the quantum properties "with your own eyes".
In 1972, Professor Leggett discovered the theoretical framework that led to the comprehension of superfluidity in Helium-3, one of the two superfluid systems known so far. Besides this theoretical breakthrough, his research has also covered a notably broad range of topics of condensed matter physics, including, for instance, pioneering contributions to the understanding of the quantum aspects of macroscopic dissipative systems, or of the Bose-Einstein condensed systems.
He also has a remarkable humanistic background and has been deeply interested in aspects concerning the foundations of physical theories, such as the completeness of the current formulations of quantum mechanics.
Since 1983 he holds the prestigious MacArthur chair at the University of Illinois at Urbana-Champaign. He is also Professor of the Centre for Advanced Studies at the same university. Some of the many important prizes that he has won during his career include the Fritz London Prize (IUPAP, 1981) or the Wolf Prize in Physics (2003). He was knighted (KBE) by Queen Elizabeth II in 2004 "for services to physics." Professor Leggett continues to maintain a fruitful research activity, as demonstrated for example by the recent studies on the possibility of magnetic order of spins in two-dimensional systems (2011).
