Floquet-engineering magnonic NOON states with performance improved by soft quantum control

We present a high-performance scheme of soft quantum control to deterministically generate entangled NOON states between two magnon modes. A superconducting qubit is considered as a data bus coupled to two magnon modes and a circuit resonator that accounts for providing an N-photon Fock state. Assisted by the Floquet engineering, effective couplings among the photon mode in the resonator and two magnon modes are constructed and can induce a chiral Fock-state transfer from the photon mode to one magnon mode, for which the magnonic Fock-state receiver mode is dependent of the qubit state. Therefore, a superposed qubit state can exactly lead to a magnonic NOON state. Compared with the recent scheme of generating magnonic NOON states (Qi and Jing in Phys Rev A 107:013702, 2023) that uses constant effective couplings, the present scheme designs Gaussian-type soft control of effective couplings, which observably improves the fidelity and robustness of generating the magnonic NOON state.