Transmission phase shifts of Kondo impurities

We study the coherent properties of transmission through Kondo impurities by considering an open Aharonov-Bohm ring with an embedded quantum dot. We develop a many-body scattering theory which enables us to calculate the conductance through the dot G _d, the transmission phase shift φ _t, and the normalized visibility η in terms of the single-particle T matrix. For the single-channel Kondo effect, we find at temperatures much below the Kondo temperature T _K that φ _t=π/2 without any corrections up to order ⁽T ^/T _K ⁾2. The visibility has the form η=1- ⁽πT ^/T _K ⁾2. For the non-Fermi-liquid fixed point of the two-channel Kondo, we find that φ _t=π/2 despite the fact that a scattering phase shift is not defined. The visibility is η=1/2(1+4λ√πT) with λ∼1/√T _K, thus, at zero temperature, exactly half of the conductance is carried by single-particle processes, and coherent transmission may actually increase with temperature. We explain that the spin summation masks the inherent scattering phases of the dot, which can be accessed only via a spin-resolved experiment. In addition, we calculate the effect of magnetic field and channel anisotropy, and generalize to the k-channel Kondo case.