  Superconducting (or Andreev) spin qubits have recently emerged as an
alternative qubit platform with realizations in semiconductor-superconductor
hybrid nanowires. In these qubits, the spin degree of freedom is intrinsically
coupled to the supercurrent across a Josephson junction via the spin-orbit
interaction, which facilitates fast, high-fidelity spin readout using circuit
quantum electrodynamics techniques. Moreover, this spin-supercurrent coupling
has been predicted to facilitate inductive multi-qubit coupling. In this work,
we demonstrate a strong supercurrent-mediated coupling between two distant
Andreev spin qubits. This qubit-qubit interaction is of the longitudinal type
and we show that it is both gate- and flux-tunable up to a coupling strength of
178 MHz. Finally, we find that the coupling can be switched off in-situ using a
magnetic flux. Our results demonstrate that integrating microscopic spin states
into a superconducting qubit architecture can combine the advantages of both
semiconductors and superconducting circuits and pave the way to fast two-qubit
gates between remote spins.
