Photon-mediated long-range coupling of two Andreev pair qubits

When two superconductors are separated by a weak link, a supercurrent is carried by Andreev bound states formed by the phase-coherent reflection of electrons and their time-reversed partners. The two levels associated with a single, highly transmissive Andreev bound state can serve as a qubit due to the potentially large energy difference with the next bound state. Although coherent manipulation of these so-called Andreev pair qubits has been demonstrated, long-range qubit-qubit coupling, which is necessary for advanced quantum computing architectures, has not yet been achieved. Here, we demonstrate coherent remote coupling between two Andreev pair qubits mediated by a microwave photon in a superconducting cavity coupler. The latter hosts two modes that are engineered to have very different coupling rates to an external port. The strongly coupled mode can be used to perform a fast read-out of each qubit, while we use the weakly coupled mode to mediate the coupling between the qubits. When both qubits are tuned into resonance with the latter mode, we find excitation spectra with characteristic avoided crossings. We identify two-qubit states that are entangled over a distance of 6 mm. This work establishes Andreev pair qubits as a compact and scalable approach to developing quantum computers. Qubits formed from Andreev bound states in a Josephson junction could have performance advantages over existing superconducting qubits. Here proof-of-principle experiments demonstrate long-range coupling between Andreev-level qubits.