Tunable single-photon scattering and bound states via the magnonic Barnett effect in giant atom-waveguide QED

We study the single-photon scattering process and the single-excitation bound states in a waveguide QED system where a giant atom (GA) couples simultaneously to a one\penalty1000-\hskip0ptdimensional coupled-resonator waveguide and an yttrium iron garnet (YIG) sphere. For a stationary YIG sphere, we demonstrate that the magnon-atom coupling strength provides flexible control over the transmission spectrum, enabling magnon\penalty1000-\hskip0ptdriven single\penalty1000-\hskip0ptphoton transport and bound states. When the YIG sphere spins, the resulting Barnett effect induces tunable photon routing: Incident photons are fully transmitted (reflected) when the bias magnetic field is parallel to the YIG sphere's angular velocity, whereas they are entirely reflected (transmitted) when the bias magnetic field is antiparallel to the YIG sphere's angular velocity. By increasing the separation between the GA's coupling points, we further realize a single\penalty1000-\hskip0ptphoton switch with either broad operational bandwidth or multiple operational frequencies. We also show that large detuning between the GA and the magnon mode suppresses the effect of the magnetically controlled single-photon switch via effective dispersive coupling, and we identify magnon\penalty1000-\hskip0ptcontrolled bound states that facilitate directional quantum storage. Finally, we simulate the dynamics of single-photon scattering, analyze the time evolution of magnon excitation and the atomic population, and discuss the dependence of magnon excitation on the magnon-atom coupling strength, Barnett frequency shift, and wave vector of the single-photon wave packet. Our work opens a way for realizing a magnon-modulated quantum single-photon switch and has potential applications in quantum engineering and scalable quantum networks.