  The entanglement transfer from electrons localized in a pair of quantum dots
to circularly polarized photons is governed by optical selection rules,
enforced by conservation of angular momentum. We point out that the transfer
can not be achieved by means of unitary evolution unless the angular momentum
of the two initial qubit states differs by 2 units. In particular, for
spin-entangled electrons the difference in angular momentum is 1 unit -- so the
transfer fails. Nevertheless, the transfer can be successfully completed if the
unitary evolution is followed by a measurement of the angular momentum of each
quantum dot and post-processing of the photons using the measured values as
input.
