A Celebration of Physics

On 28 March, ICTP celebrated excellence in physics with a ceremony honouring the three recipients of its top prize, the Dirac Medal.

Three distinguished physicists shared the 2018 award: Subir Sachdev of Harvard University, Dam Thanh Son of the University of Chicago, and Xiao-Gang Wen of the Massachusetts Institute of Technology. The award cites their independent contributions toward understanding novel phases in strongly interacting many-body systems, introducing original cross-disciplinary techniques.

In a ceremony held in ICTP's Budinich Lecture Hall, the three medallists received their prizes and delivered lectures. David Tong from the University of Cambridge, UK, provided a lively introduction to the three medallists and their work. The event was recorded for ICTP's international audience. Photos of the ceremony are available on ICTP's Flickr page.

All three winners study how quantum mechanics affects large groups of particles, known as many-body systems. Researchers now understand how the laws of quantum mechanics affect the behavior of very small groups of particles, but everyday objects are made up of a huge amount of particles, nearly 10²³. All of the particles are interacting with each other in various ways. These interactions make quantum entanglement important to consider, and so applying quantum mechanics to these systems becomes very complicated. The complex patterns of quantum entanglement are key to understanding the macroscopic properties of a material, especially when many-body systems exhibit surprising emergent behavior.

Some of this emergent behaviour results in novel phases of matter. One of the ways to study many-body systems is to look at the phase a material is in. The familiar phases are solid, liquid, or gas, but with modern quantum materials, many new phases of matter have been discovered, with new properties that have intrigued scientists. The 2018 Dirac Medal winners have made pioneering contributions to understanding them, as well as the "phase transitions," i.e. passing from one phase to another, that occur when external parameters like temperature and pressure are slowly changed. Phase changes mean the properties of a material can drastically change, and the three medalists have helped elucidate how patterns of electron entanglement can give rise to these property changes.

Understanding the dynamics of many-body systems allows scientists to understand how different properties of materials emerge, not to mention help in the effort to design new types of materials. These materials could potentially be used for applications from new quantum computers to superconducting devices. The Dirac Medallists  have used their broad knowledge of areas including materials science, black holes, and cold atoms to further our understanding of many-body systems, proving the value of a multidisciplinary approach.

"This year's Dirac Medallists are all leaders in using cross-disciplinary methods for concrete theoretical physics questions," said ICTP Director Fernando Quevedo.  He added that the three winners are good role models for the thousands of scientists who visit ICTP from developing countries. "Even though the Medallists live in the US, I am pleased that they are all from developing countries and have been close to ICTP and its mission."

Subir Sachdev, born in New Delhi, India, has made pioneering contributions to many areas of theoretical condensed matter physics. Of particular importance were the development of the theory of quantum critical phenomena in insulators, superconductors and metals; the theory of spin-liquid states of quantum antiferromagnets and the theory of fractionalized phases of matter; the study of novel deconfinement phase transitions; the theory of quantum matter without quasiparticles; and the application of many of these ideas to a priori unrelated problems in black hole physics, including a concrete model of non-Fermi liquids.

Dam Thanh Son, born in Hanoi, Vietnam, was the first to understand that gauge/gravity duality could be used to address basic questions in strongly interacting many-body problems from cold trapped atoms to the quark-gluon plasma. He was able to show that one could compute transport coefficients, such as viscosity and conductivity, analytically in these systems, and that strong coupling typically gives rise to a bound on these coefficients. More recently, he has argued for the emergence of a Dirac fermion at the half-filled Landau level, work which has stimulated rapid developments in our understanding of three-dimensional gauge theories.

Xiao-Gang Wen, born in Beijing, China, has pioneered the concept of topological order as a new principle to understand gapped quantum systems. He found that states with topological order contain non-trivial boundary excitations, and he developed chiral Luttinger theory for the boundary states of quantum Hall systems. He realized that quantum Hall states fall outside of the usual Landau paradigm for characterizing phases of matter, and he showed how to classify them.  He unveiled deep connections between topological order and entanglement. More recently, he has developed the concept of symmetry protected topological phases. These ideas have close connections to anomalies in quantum field theory.

ICTP's Dirac Medal, first awarded in 1985, is given in honor of P.A.M. Dirac, one of the greatest physicists of the 20th century and a staunch friend of the Centre. It is awarded annually on Dirac's birthday, 8 August, to scientists who have made significant contributions to theoretical physics. An award ceremony, during which the three winners will present lectures on their work, will take place later this year. For more details about the prize and a list of past winners, visit the web page.