  We consider two quantum dots described by the Anderson-impurity model with
one electron per dot. The goal of our work is to study the decay of a maximally
entangled state between the two electrons localized in the dots. We prepare the
system in a perfect singlet and then tunnel-couple one of the dots to leads,
which induces the non-equilibrium dynamics. We identify two cases: if the leads
are subject to a sufficiently large voltage and thus a finite current, then
direct tunneling processes cause decoherence and the entanglement as well as
spin correlations decay exponentially fast. At zero voltage or small voltages
and beyond the mixed-valence regime, virtual tunneling processes dominate and
lead to a slower loss of coherence. We analyze this problem by studying the
real-time dynamics of the spin correlations and the concurrence using two
techniques, namely the time-dependent density matrix renormalization group
method and a master-equation method. The results from these two approaches are
in excellent agreement in the direct-tunneling regime for the case in which the
dot is weakly tunnel-coupled to the leads. We present a quantitative analysis
of the decay rates of the spin correlations and the concurrence as a function
of tunneling rate, interaction strength, and voltage.
