CAM Colloquium-- Peter Littlewood, University of Chicago Department of Physics "Non-reciprocal phase transitions"


655 Rhodes Hall


Abstract: Spontaneous synchronization is at the core of many natural phenomena. Your heartbeat is maintained because cells contract in a synchronous wave; some bird species synchronize their motion into flocks; quantum synchronization is responsible for laser action and superconductivity. The transition to synchrony, or between states of different patterns of synchrony, is a dynamical phase transition that has much in common with conventional phase transitions of state – for example solid to liquid, or magnetism – but the striking feature of driven dynamical systems is that the components are “active”. Consequently quantum systems with dissipation and decay are described by non-Hermitian Hamiltonians, and active matter can abandon Newton’s third law and have non-reciprocal interactions. This substantially changes the character of many-degree-of-freedom dynamical phase transitions between steady states and the critical phenomena in their vicinity, since the critical point is an “exceptional point” where eigenvalues become degenerate and eigenvectors coalesce.

We will illustrate this in several different systems – a Bose-Einstein condensate of polaritons, models of multicomponent active matter such as flocks of birds, generalized Kuramoto models, models of neurons, and an ancient example from Huygens. We argue that there is a systematic theory and generalized phase diagram, and corresponding universality behaviors determined by the symmetry of the models.

Bio: Littlewood holds a BA and PhD in Physics from the University of Cambridge. He was member of technical staff, and later head, of the theoretical physics research group at Bell Laboratories in New Jersey. He moved to Cambridge in 1997 as head of the Theory of Condensed Matter group, and later became head of the Cavendish Laboratory and Department of Physics. He came to UChicago in 2011 as Associate Lab Director and then Lab Director, at Argonne National Lab, returning full time to the University in 2017. He serves on the advisory boards of several institutes, including the Faraday Institution, the Simons Foundation, the Paul Scherer Institute, the Carnegie Institute for Science, and the Max Planck Institutes at Halle and Hamburg.