Universality in the ferromagnetic strong coupling regime of the Kondo model out of equilibrium*

We present a nonperturbative method for calculating the time-dependent many body wavefunction following a local quench and use it to find new results in and out of equilibrium in the universal strong ferromagnetic regime of the Kondo model. The method addresses the general problem of calculating e^{-i H t} Ψ, where the initial state Ψ is a free N-particle state (such as a Fermi sea) and H is a many body Hamiltonian which is switched on at t=0. While the method may be of wider applicability, we have so far focused on one dimensional models of quantum dots attached to linearized leads. The method yields the exact many body wavefunction of the two lead Kondo model following the quench. In the strong ferromagnetic regime, we find a new universal scale T_K = D exp[3π² ρ J / 8] and the surprising result that the steady state conductance dI/dV reaches the unitarity limit asymptotically at large voltage or temperature.