  Entangling two quantum bits by letting them interact is the crucial
requirements for building a quantum processor. For qubits based on the spin of
the electron, these two-qubit gates are typically performed by exchange
interaction of the electrons captured in two nearby quantum dots. Since the
exchange interaction relies on tunneling of the electrons, the range of
interaction for conventional approaches is severely limited as the tunneling
amplitude decays exponentially with the length of the tunneling barrier. Here,
we present a novel approach to couple two spin qubits via a superconducting
coupler. In essence, the superconducting coupler provides a tunneling barrier
for the electrons which can be tuned with exquisite precision. We show that as
a result exchange couplings over a distance of several microns become
realistic, thus enabling flexible designs of multi-qubit systems.
