This year's lecture concerns quantum systems in which low-energy excitations cannot be described in terms of quasiparticles. The transport properties of these systems are unusual and different from a usual Fermi liquid ("strange metal"), with, for example, a resistivity that depends linearly on temperature. Dissipation in these systems is "Planckian", i.e. as fast as quantum mechanics allows. The lecture will focus on a class of models introduced by Sachdev and Ye, then Kitaev (SYK) and their generalizations, which allow us to understand these phenomena within a precise theoretical framework. In addition, a remarkable correspondence has been established between these models and certain theories of quantum gravitation. These aspects will be discussed in four guest lectures by Professor Subir Sachdev (Harvard University).
