Prof. Shivaji Sondhi is professor of physics at Princeton University, USA. His research has focused on materials in which the interactions among the electrons are important, even for a qualitative understanding of their behavior. This is in contrast to the textbook examples of metals and insulators, whose behavior is largely explicable in terms of independent electrons.The particular systems he has worked on are the two-dimensional electron gases that are realized in semiconductor heterostructures, superconducting fullerides, frustrated magnetic systems and the cuprate superconductors. Current interests include: "Quantum Hall" systems and searching for possible new phases in the quantum Hall regime, and the more theoretical question of constructing "better" field theories of the high-field dynamics; as well as continuous quantum phase transitions. Abstract: A population can be immune to epidemics even if not all of its individual members are immune to the disease, just as long as sufficiently many are immune -- this is the traditional notion of herd immunity. In the smartphone era a population can be immune to epidemics even if not a single one of its members is immune to the disease -- a notion we propose to call "digital herd immunity", which is similarly an emergent characteristic of the population. This immunity arises because contact-tracing protocols based on smartphone capabilities can lead to highly efficient quarantining of infected population members and thus the extinguishing of nascent epidemics. When the disease characteristics are favorable and smartphone usage is high enough, the population is in this immune phase. As usage decreases there is a novel "contact tracing" phase transition to an epidemic phase. We present and study a simple branching-process model for COVID-19 and show that digital immunity is possible regardless of the proportion of non-symptomatic transmission. We believe this is a promising strategy for dealing with COVID-19 in many countries. The talk (approx. 40 minutes) will be in a webinar format, with an introduction by the Director. A question/answer session will follow. PLEASE REGISTER AHEAD FOR THIS WEBINAR AT THE FOLLOWING: https://zoom.us/webinar/register/WN_HPsW1arNSxKdLvHs8p7Uzg After registering, you will receive a confirmation email containing information about joining the webinar.