Analog quantum control of magnonic cat states on-a-chip by a superconducting qubit

We propose to directly and quantum-coherently couple a superconducting transmon qubit to magnons – the quanta of the collective spin excitations, in a nearby magnetic particle. The magnet’s stray field couples to the qubit via a superconducting interference device (SQUID). We predict a resonant qubit-magnon exchange and a nonlinear radiation-pressure interaction that are both stronger than dissipation rates and tunable by an external flux bias. We additionally demonstrate a quantum control scheme that generates qubit-magnon entanglement and magnonic Schrödinger cat states with high fidelity.