Nonreciprocity and Unconventional Singularities in Cavity Magnonics INVITED

Abstract

Cavity magnonics built on strongly coupled cavity photons and magnons has gradually demonstrated its unique advantages in fundamental and applied research in the past few years. It has several advantages, including flexible turnability, excellent controllability, and high compatibility. Furthermore, a versatile platform based on coherently coupled microwave photons, magnons, phonons, superconducting qubits, and optical photons has begun to take shape. The hybrid system is expected to be applied in quantum information processing, transfer, and storage. Before 2018, all experiments were carried out in the case of coherent coupling between cavity photons and magnons. In 2018, an intriguing dissipative coupling between cavity photon and magnon was observed for the first time, which originated from the cooperative dissipation of the cavity mode and magnon mode. Unlike the coherent coupling case, where the spectrum is shown as an anti-crossing between cavity mode and magnon mode, the spectrum is shown as a level attraction in the case of dissipative coupling. The revelation of dissipative coupling greatly enhances the richness and functionality of the cavity magnonics and stimulates some fundamental research. We will introduce this trend with two recent works from our group.After revealing the dissipative coupling mechanism, we first realized nonreciprocity and unidirectional invisibility in cavity magnonics by utilizing the interference effect between the coherent and dissipative couplings. The isolation ratio can be relatively large with optimized small insertion loss.With the dissipative coupling mechanism and magnon mode linewidth engineering technique, we construct an anti-parity-time (APT) symmetric cavity magnonics system. Besides the dissipative coupling, the APT symmetric system also needs (i) the cavity mode and magnon mode have the opposite frequency, which can be effectively realized with a detuned cavity magnonics system in the rotating frame, (ii) the same damping or gain rates of the cavity mode and magnon mode, which can be engineered by adjusting the damping rate of the magnon mode to match that of the cavity mode. Further, we observed the exceptional points in this APT symmetric cavity magnonics system, which are the transition points between the PT-symmetry-preserved and PT-symmetry-broken phases. Between two EPs, the maximal coherent superposition of photon and magnon states is robustly sustained by the preserved APT symmetry. Moreover, we observed another kind of singularity, arising from the dissipative coupling of two antiresonances, is an unconventional bound state in the continuum (BIC). At the settings of BICs, the coupled system exhibits infinite discontinuities in the group delay. We find that both singularities coexist at the equator of the eigenvector mapped Bloch sphere, revealing a unique hybrid state that simultaneously exhibits the maximal coherent superposition and slow light capability. We expect to explore more interesting physics and applications based on the dissipative coupling in cavity magnonics. **