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 has been 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.