  We consider a triple quantum dot system in a triangular geometry with one of
the dots connected to metallic leads. Using Wilson's numerical renormalization
group method, we investigate quantum entanglement and its relation to the
thermodynamic and transport properties, in the regime where each of the dots is
singly occupied on average, but with non-negligible charge fluctuations. It is
shown that even in the regime of significant charge fluctuations the formation
of the Kondo singlets induces switching between separable and perfectly
entangled states. The quantum phase transition between unentangled and
entangled states is analyzed quantitatively and the corresponding phase diagram
is explained by exactly solvable spin model.
