A quantum impurity is one particle, or more generally, a discrete quantum system, which interacts with a large ensemble of other particles (or a continuous quantum system). The particles in the bath do not interact with each other (or interact very weakly) so the only relevant interaction is between the bath and the impurity. Quantum impurities are simpler to describe than general quantum many-body systems, where each particle can interact with a large number of other particles. Nevertheless, they exhibit a lot of the fascinating effects that we see in the latter. This makes them fundamentally interesting and also an ideal testing ground for new numerical (and analytical) techniques.

Together with Guy Cohen and Moshe Goldstein I am studying non-equilibrium properties of the so-called spin-boson model. It consists of a fixed spin (a two-state system) coupled to a bath of non-interacting harmonic modes (bosons). It can be used to model dissipation in quantum systems, but also interactions with photons or phonons, as well as electronic systems via bosonization. We are interested in non-equilibrium and dynamical properties of the spin-boson model, for example how the occupation of the two states changes over time. We can access these properties with numerically exact Quantum Monte Carlo methods, specifically the inchworm algorithm