  We propose a scheme for detecting entanglement between two electron spin
qubits in a double quantum dot using an entanglement witness operator. We first
calculate the optimal configuration of the two electron spins, defined as the
position in the energy level spectrum where, averaged over the nuclear spin
distribution, 1) the probability to have two separated electrons, and 2) the
degree of entanglement of the quantum state quantified by the concurrence are
both large. Using a density matrix approach, we then calculate the evolution of
the expectation value of the witness operator for the two-spin singlet state,
taking into account the effect of decoherence due to quantum charge
fluctuations modeled as a boson bath. We find that, for large interdot
coupling, it is possible to obtain a highly entangled and robust ground state.
