Effect of localized spins in coherent transport through quantum dots

The effect of localized spins on the quantum coherence in solids is discussed. A quantum dot with an odd number of electrons can be a model system for a localized spin. It is experimentally shown that a spin flip scattering by a quantum dot pulls the trigger of quantum decoherence. On the other hand, spin flip scattering is the basic process to construct the Kondo singlet state around a magnetic impurity. Through an interference effect of the Kondo state (the Fano-Kondo effect) in a side-coupled dot system, we show experimentally that the Kondo singlet state is quantum mechanically coherent. The analysis of the Fano-Kondo lineshape indicates the locking of the phase shift to π/2, which is in agreement with theoretical predictions. The Fano-Kondo effect is also observed in an Aharonov-Bohm ring, in which a quantum dot is embedded, and also indicates the phase shift locking to π/2.