Exchange interaction of two spin qubits mediated by a superconductor

Entangling two quantum bits by letting them interact is the crucial requirement 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 an 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 multiqubit systems.