  A superconductor is a natural source of spin-entangled spatially separated
electron pairs. Although the first Cooper-pair splitter devices have been
realized recently, an experimental confirmation of the spin state and the
entanglement of the emitted electron pairs is lacking up to now. In this paper
a method is proposed to confirm the spin-singlet character of individual split
Cooper pairs. Two quantum dots (QDs), each of them holding one spin-prepared
electron, serve as the detector of the spin state of a single Cooper pair that
is forced to split when it tunnels out from the superconductor to the QDs. The
number of charges on the QDs, measured at the end of the procedure, carries
information on the spin state of the extracted Cooper pair. The method relies
on the experimentally established toolkit of QD-based spin qubits: resonant
spin manipulation, Pauli blockade, and charge measurement.
