Switchable superradiant phase transition with Kerr magnons

Nonreciprocal devices, allowing to manipulate one‐way signals, are crucial to quantum information processing and quantum networks. Here a nonlinear cavity‐magnon system is proposed, consisting of a microwave cavity coupled to one or two yttrium–iron–garnet (YIG) spheres supporting magnons with Kerr nonlinearity, to investigate nonreciprocal unconventional photon blockade. The nonreciprocity originates from the direction‐dependent Kerr effect, distinctly different from previous proposals with spinning cavities and dissipative couplings. For a single sphere case, nonreciprocal unconventional photon blockade can be realized by manipulating the nonreciprocal destructive interference between two active paths, via varying the Kerr coefficient from positive to negative, or vice versa. By optimizing the system parameters, the perfect and well‐tuned nonreciprocal unconventional photon blockade can be predicted. For the case of two spheres with opposite Kerr effects, only reciprocal unconventional photon blockade can be observed when two cavity‐magnon coupling strengths Kerr strengths are symmetric. However, when coupling strengths or Kerr strengths become asymmetric, nonreciprocal unconventional photon blockade appears. This implies that two‐sphere nonlinear cavity‐magnon systems can be used to switch the transition between reciprocal and nonreciprocal unconventional photon blockades. This study offers a potential platform for investigating the nonreciprocal photon blockade effect in nonlinear cavity magnonics.

Nonreciprocal Unconventional Photon Blockade with Kerr Magnons

Fan,

Zhang,

et al. 2024

Robust Topological Feature against Non‐Hermiticity in Jaynes–Cummings Model

Ying

2024

Self Cite

The Jaynes–Cummings Model (JCM) is a fundamental model and building block for light‐matter interactions, quantum information and quantum computation. The present work analytically analyzes the topological feature manifested by the JCM in the presence of non‐Hermiticity which may arise from dissipation and decay rates. Indeed, the eigenstates of the JCM are topologically characterized by spin windings in 2D plane. The non‐Hermiticity tilts the spin‐winding plane and induces an out‐of‐plane component, while the topological feature is maintained. In particular, besides the invariant spin texture nodes, the study reveals a non‐Hermiticity‐induced reversal transition of the tilting angle and spin winding direction with a fractional phase gain at gap closing, a partially level‐independent reversal transition without gap closing, and a completely level‐independent super‐invariant point with untilted angle and also without gap closing. The result demonstrates that the topological feature is robust against non‐Hermiticity, which may be favorable in practical applications. On the other hand, one may conversely make use of the disadvantageous dissipation and decay rates to reverse the spin winding direction, which may add a controlled way for topological manipulation of quantum systems in light‐matter interactions.

Mechanical Dynamics Around Higher‐Order Exceptional Point in Magno‐Optomechanics

He,

Fan,

Liu

et al. 2024

Diverse exceptional points (EPs) are theoretically studied in an experimentally feasible magno‐optomechanics consisting of an optomechanical subsystem coupled to a magnomechanical subsystem via physically direct contact. By adiabatically eliminating both the cavity and the Kittel mode, dissipative and parity‐time symmetric exceptional points can be observed. When only the cavity mode is eliminated, a second (third)‐order pseudo‐Hermitian EP emerges for nondegenerate (degenerate) mechanical modes. The distinct dynamical behavior of two mechanical modes around these EPs are further studied. The proposal provides a promising way to engineer diverse EPs and quantify non‐Hermitian phase transition with exceptional dynamical behavior in magno‐optomechanics.