A measurable form of Bell's correlation of spin currents across a double dot in the Kondo regime*

We investigate Bell-state correlations of quasiparticle pairs that are excited within nonlinear current through a double dot in the Kondo regime. Using the renormalized perturbation expansion in the residual interaction of the local Fermi liquid, we calculate an asymptotically exact form of Bell's correlation at low energies. We find that the exchange interaction of the double dot violates Bell's inequality. We especially suggest a measurable form of Bell's inequality that is given by correlations of the full current across the double dot. We illustrate the exchange interaction dependence of upper limits of the suggested Bell's correlation in the quantum theory and the hidden variable theory, using the numerical renormalization group calculation.