  A semiconductor nanowire brought in proximity to a superconductor can form
discrete, particle-hole symmetric states, known as Andreev bound states (ABSs).
An ABS can be found in its ground or excited states of different spin and
parity, such as a spin-zero singlet state with an even number of electrons or a
spin-1/2 doublet state with an odd number of electrons. Considering the
difference between spin of the even and odd states, spin-filtered measurements
have the potential to reveal the underlying ground state. To directly measure
the spin of single-electron excitations, we probe an ABS using a spin-polarized
quantum dot that acts as a bipolar spin filter, in combination with a
non-polarized tunnel junction in a three-terminal circuit. We observe a
spin-polarized excitation spectrum of the ABS, which in some cases is fully
spin-polarized, despite the presence of strong spin-orbit interaction in the
InSb nanowires. In addition, decoupling the hybrid from the normal lead blocks
the ABS relaxation resulting in a current blockade where the ABS is trapped in
an excited state. Spin-polarized spectroscopy of hybrid nanowire devices, as
demonstrated here, is proposed as an experimental tool to support the
observation of topological superconductivity.
