We realize a cavity magnon-microwave photon system in which magnetic dipole interaction mediates strong coupling between collective motion of large number of spins in a ferrimagnet and the microwave field in a three-dimensional cavity. By scaling down the cavity size and increasing number of spins, an ultrastrong coupling regime is achieved with a cooperativity reaching 12600. Interesting dynamic features including classical Rabi oscillation, magnetically induced transparency, and Purcell effect are demonstrated in this highly versatile platform, highlighting its great potential for coherent information processing. Introduction.-Systems with strong light-matter interaction have played crucial roles in quantum [1,2] and classical information processing [3,4] as they enable coherent information transfer between distinct physical platforms. It is well known that systems with large electric dipole moment can couple strongly with the optical fields. However, the possibility of strong light-matter interaction via magnetic dipoles is mostly ignored. It is only recently that Imamoglu [5] has pointed out the direction to achieve strong light-matter interaction using collective excitations of spin ensembles, and visioned the promise of quantum information processing in these systems. Since then, various implementations have been proposed and experimentally investigated. Ensembles including ultracold atomic clouds [6], molecules [7], nitrogen vacancy centers in diamond [8][9][10][11][12][13], and ion doped crystals [14][15][16] have been used to couple to microwave resonators or even superconducting qubits.

Extensive efforts have been expended in developing hybrid quantum systems to overcome the short coherence time of superconducting circuits by introducing the naturally long-lived spin degree of freedom. Among all the possible materials, single-crystal yttrium iron garnet has shown up recently as a promising candidate for hybrid systems, and various highly coherent interactions, including strong and even ultrastrong coupling, have been demonstrated. One distinct advantage in these systems is that spins form well-defined magnon modes, which allows flexible and precise tuning. Here we demonstrate that by dissipation engineering, a non-Markovian interaction dynamics between the magnon and the microwave cavity photon can be achieved. Such a process enables us to build a magnon gradient memory to store information in the magnon dark modes, which decouple from the microwave cavity and thus preserve a long lifetime. Our findings provide a promising approach for developing long-lifetime, multimode quantum memories.

Single-crystal organic nanowires were fabricated with a soft-template-assisted self-assembly method in liquid phase. These nanowires with rectangular cross section can serve as resonators for exciton-photon coupling, leading to a microcavity effect and a relatively low threshold of laser actions. Two-photon-pumped blue lasing was observed in these organic waveguiding nanostructures above a threshold of 60 nJ, excited with a 750 nm near-infrared femtosecond pulse laser at 77 K.