Introduction to mesoscopic physics : electrons and photons (2)

In the second lesson, we examined how electron fluid screens the charge of individual electrons and arrived at the notion of quasi-particles, low-energy fermionic excitations of electron fluid that obey the Pauli exclusion principle and carry a transverse charge equal to that of the electron, while having a strongly screened longitudinal charge over a radius of the order of the average distance between electrons. Unlike bare   electrons, quasi-electrons have a finite lifetime, as they can " disintegrate " into a lower-energy electron accompanied by an electron-hole pair, or more generally, an excitation of bosonic character. This led us to discuss the plasmonic modes of a metal wire and their relationship to ordinary photons in atomic physics. The picture that emerges from examining the elementary excitations considered in mesoscopic physics, whether fermionic or bosonic, is that of effective particles dressed up by their interactions with many degrees of freedom. But while we lose the particle purity of atomic physics, we gain remarkable flexibility in implementing and controlling the interactions between these effective particles. This introduces the subject of the next lecture : it will focus on quantum circuits and signals, and deal with bosonic mesoscopic systems in which interactions between effective photons are induced by the non-linearity of Josephson tunnel junctions.