  Serial double quantum dots created in semiconductor nanostructures provide a
versatile platform for investigating two-electron spin quantum states, which
can be tuned by electrostatic gating and an external magnetic field. In this
work, we directly measure the supercurrent reversal between adjacent charge
states of an InAs nanowire double quantum dot with superconducting leads, in
good agreement with theoretical models. In the even charge parity sector, we
observe a supercurrent blockade with increasing magnetic field, corresponding
to the spin singlet to triplet transition. Our results demonstrate a direct
spin to supercurrent conversion, the superconducting equivalent of the Pauli
spin blockade. This effect can be exploited in hybrid quantum architectures
coupling the quantum states of spin systems and superconducting circuits.
