  The operation of a source of entangled electron spins, based on a
superconductor and two quantum dots in parallel\cite{loss}, is described in
detail with the help of quantum master equations. These are derived including
the main parasitic processes in a fully consistent and non-perturbative way,
starting from a microscopic Hamiltonian. The average current is calculated,
including the contribution of entangled and non-entangled pairs. The
constraints on the operation of the device are illustrated by a calculation of
the various charge state probabilities.
