Physics Colloquium

Small random fluctuations, either of thermal or quantum
origin, are the cause of many important and interesting physical phenomena. These include chemical reactions, nucleation in phase transitions (i.e., the formation of a droplet of one phase within another phase), and the formation of unusual spatially localized states in various condensed matter systems. In all of these, random fluctuations (or `noise'), no matter how small, eventually drives a physical system from one stable state to another.

We consider both classical and quantum noise, and discuss how thermally activated hopping over a barrier crosses over to quantum tunneling through a barrier as temperature is lowered. This crossover has some (but not all) features of a second-order phase transition. We also discuss two timely applications from mesoscopic physics: thermally induced breakup of monovalent metallic nanowires, and stochastic reversal of magnetization in thin ferromagnetic
annuli. Each are of interest both from the point of view of
fundamental physics and for potential technological applications.