This paper demonstrates a four-qubit quantum gate, where two qubits control operations on two target qubits. This configuration implements four controlled two-qubit gates: two entangling swap and phase operations, a parity-distinguishing phase operation, and an identity operation. The device uses quantum interference, and connecting multiple such devices creates a highly connected quantum computer. Superconducting transmon qubits in a diamond-shaped architecture are proposed for implementation, achieving fidelities around 0.99 in under 100 ns.

Niels Jakob Søe Loft, Morten Kjaergaard, Lasse Bjørn Kristensen, Christian Kraglund Andersen, Thorvald W. Larsen, Simon Gustavsson, William D. Oliver, Nikolaj T. Zinner