Realistic operation of an entangler : a density matrix approach

4 pages, 4 figures. To appear in the proceedings of the XXXIXth Rencontres de Moriond (La Thuile, 2004) "Quantum information and decoherence in nanosystems"The detailed operation of an electron spin entangler is studied, using density matrix equations. The device is made of a superconductor, two quantum dots and two normal leads. The treatment takes into account coherent tunneling in a non-perturbative way, and analyzes the various parasitic effects, in addition to the main process (crossed Andreev reflection) : those include singlet pairs passing through a single dot, or cotunneling between dots through the superconductor. The optimum operation of the device is characterized