# Nonlinear I-V Curve at a Quantum Critical Point and Quantum Noise

Many-body systems that are driven far from equilibrium or simply subjected to quantum noise exhibit complex interplay between the many-body correlations and the external variables, and so are attracting increasing attention. I shall discuss a system that is particularly advantageous for studying these effects: it exhibits impurity quantum criticality, it is amenable to detailed experimental study (and initial experiments have been done), and it is simple enough theoretically that analytical results can be obtained. First, I briefly survey the experimental system and initial results. The system consists of a spin-polarized carbon nanotube quantum dot connected to resistive leads. A quantum critical point (QCP) occurs when a level in the dot is resonant with the leads and the dot is symmetrically coupled to them. (ii) Second, I present our calculation of the nonlinear I-V curve at the QCP and show remarkable agreement with the experiment. This result has a simple interpretation as an environmental blockade.