Implementation of pairwise longitudinal coupling in a three-qubit superconducting circuit

Abstract

We present the “trimon,” a multimode superconducting circuit implementing three qubits with all-to-all longitudinal coupling. This always-on interaction enables simple implementation of generalized controlled-not gates which form a universal set. Furthermore, two of the three qubits are protected against Purcell decay while retaining measurability. We demonstrate high-fidelity state swapping operations between two qubits and characterize the coupling of all three qubits to a neighboring transmon qubit. Our results offer a different paradigm for multiqubit architecture with applications in quantum error correction, quantum simulations, and quantum annealing.