Strong Coupling of an Fe - Co Alloy with Ultralow Damping to Superconducting Co-planar Waveguide Resonators

Abstract

We report on the strong coupling between a metallic ferromagnetic Fe75Co25 thin film patterned element and a range of superconducting Nb half-wavelength co-planar waveguide (CPW) resonators. By varying the volume of the ferromagnet we demonstrate that the coupling rate scales linearly with the square root of the number of spins and achieve a coupling rate over 700 MHz, approaching the ultrastrong coupling regime. Experiments varying the center conductor width while maintaining constant magnetic volume verify that decreasing the center conductor width increases coupling and cooperativity. Our results show that the frequency dependence of the coupling rate is linear with the fundamental and higher order odd harmonics of the CPW, but with differing efficiencies. The results show promise for scaling planar superconducting resonator/magnetic hybrid systems to smaller dimensions. Quantum technologies based on hybrid systems where light is strongly coupled to a degree of freedom in a solid state system have the potential to overcome some of the practical engineering challenges limiting large-scale quantum computing. A hybrid system that has been recently investigated is based on the interaction between resonant microwave photons and the collective spin excitations in a magnetic structure. Experiments exploiting the coherent coupling between the two systems have demonstrated strong and ultra-strong coupling regimes, magnetically induced transparency, the Purcell effect, cavity mediated spin-spin coupling, and even potential quantum memories. Dissipative coupling has been used to explore frequency attraction and investigate the non-Hermitian physics possible in such systems. At millikelvin temperatures, strong coupling between magnons and a qubit, mediated by a cavity, have been used to measure individual magnon and photon numbers. Other experiments have demonstrated the potential for optical-to-microwave transduction, a critical technology for quantum information systems (QIS), using magneto-optical coupling between optical and spin-wave modes. Typically, the microwave resonator used in these experiments is a three-dimensional cavity and the magnetic element is a highly polished yttrium iron garnet (YIG, Y3Fe5O12) sphere, however other geometries have also been explored. YIG is an excellent material due to its low damping, which allows for long magnon lifetimes and microwave cavities with high quality factors (Q) are readily available allowing for long photon lifetimes. While this specific configuration has been used with much success, many promising quantum computing platforms are microfabricated, and it is likely that any scalable quantum system will largely be lithographically defined to efficiently scale, and utilize planar signal lines and resonators such as the co-planar-waveguide (CPW) resonator. Consequently, it is likely beneficial to have a similar lithographically defined system for hybrid magnetic systems. The smallest commercially available YIG spheres are on the order of 200 μm in diameter and would likely be incompatible with such a system.