Observation of the exceptional point in cavity magnon-polaritons

Abstract

Magnon–polaritons are hybrid light–matter quasiparticles originating from the strong coupling between magnons and photons. They have emerged as a potential candidate for implementing quantum transducers and memories. Owing to the dampings of both photons and magnons, the polaritons have limited lifetimes. However, stationary magnon–polariton states can be reached by a dynamical balance between pumping and losses, so the intrinsically nonequilibrium system may be described by a non-Hermitian Hamiltonian. Here we design a tunable cavity quantum electrodynamics system with a small ferromagnetic sphere in a microwave cavity and engineer the dissipations of photons and magnons to create cavity magnon–polaritons which have non-Hermitian spectral degeneracies. By tuning the magnon–photon coupling strength, we observe the polaritonic coherent perfect absorption and demonstrate the phase transition at the exceptional point. Our experiment offers a novel macroscopic quantum platform to explore the non-Hermitian physics of the cavity magnon–polaritons.

Steering between level repulsion and attraction: broad tunability of two-port driven cavity magnon-polaritons

“… symmetric systems are studied in many different fields such as in quantum mechanics [41], optical microcavities [42] or magnetism and magnonics [43]. This symmetry also started to receive interest in cavity spintronics and for CMPs where the spectra and behaviour of  symmetric CMPs have recently been discussed [20,44,45].…”

“…However, in most of these works, being able to control the coupling constant is imperative; whether the ultimate goal is to achieve stronger coupling or to control the actual state of coupling [20]. While most of the aforementioned initial studies have concentrated in the case of level repulsion , leading to said avoided level crossing-also known as Rabi splitting [21]-more recently, another phenomenon has emerged : the so-called 'level attraction' [22][23][24][25][26].…”

Entangling two magnon modes via magnetostrictive interaction

“…This strong coupling offers a possibility to enable coherent information transfer between drastically different information carriers, and thus may find potential applications in quantum information processing, especially when the system becomes hybrid [10], such as by coupling magnons to a superconducting qubit [11,12], to phonons [13,14], or to both microwave and optical photons [15]. Furthermore, various interesting phenomena have been explored in the system of cavity-magnon polaritons, such as the observation of magnon gradient memory [16], the exceptional point [17,18], manipulation of distant spin currents [19], and bistability [20], to name but a few.…”

Level attraction and level repulsion of magnon coupled with a cavity anti-resonance

“…Although the coupling strength between a single spin and a microwave photon is very weak, stronger coupling can be achieved by the use of collective excitations (also called as magnons) in ferrimagnetic materials such as yttrium iron garnet (YIG, Y 3 Fe 5 O 12 ) [4][5][6][7][8][9]. Various interesting phenomena, including the manipulation of spin current [10,11], the magnon quintuplet state [12], the exceptional point [13] and the bistability of cavity magnon polaritons [14] have been demonstrated in strongly coupled magnonphoton systems. In addition to the fundamental uniform mode, coupling between photons and higher order non-uniform modes has also been studied, with coupling strength depending on the overlap between the magnon and caity modes [15,16].…”