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arXiv:2606.25511

Preparing two-mode magnonic Schrödinger cat states in a cavity-magnon-qubit system

Gen Li, Gang Liu, Rong-Can Yang, Jie Li

quant-phcond-mat.mes-hallphysics.opticsSubmitted Jun 24, 2026• Announced Jun 25, 2026• 0 citations
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Abstract

The cavity-magnon-qubit system has recently been demonstrated as a new platform for preparing macroscopic quantum states in magnonic systems. Here, we propose to prepare a two-mode magnonic cat state, which is also a non-Gaussian entangled state, based on this practical system involving two yttrium-iron-garnet (YIG) spheres and a superconducting qubit coupled to a common microwave cavity. By adiabatically eliminating the cavity and resonantly driving the qubit, an effective magnon-qubit conditional-displacement interaction is achieved. Further working in the magnon-magnon strong-coupling regime and considering two identical magnon frequencies and coupling strengths to the cavity, two hybridized magnon modes are formed, of which the bright mode is prepared in a cat state after a projective measurement on the qubit, while the dark mode remains in its initial vacuum state. Such a state corresponds to a two-mode cat state of two original magnon modes, which share strong non-Gaussian entanglement. We also discuss practical dissipation and dephasing effects on the cat state. The results indicate that strong nonclassicality and non-Gaussian entanglement are present in the two-mode cat state using fully feasible parameters.

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The cavity-magnon-qubit system has recently been demonstrated as a new platform for preparing macroscopic quantum states in magnonic systems. Here, we propose to prepare a two-mode magnonic cat state, which is also a non-Gaussian entangled state, based on this practical system involving two yttrium-iron-garnet (YIG) spheres and a superconducting qubit coupled to a common microwave cavity. By adiabatically eliminating the cavity and resonantly driving the qubit, an effective magnon-qubit conditional-displacement interaction is achieved. Further working in the magnon-magnon strong-coupling regime and considering two identical magnon frequencies and coupling strengths to the cavity, two hybridized magnon modes are formed, of which the bright mode is prepared in a cat state after a projective measurement on the qubit, while the dark mode remains in its initial vacuum state. Such a state corresponds to a two-mode cat state of two original magnon modes, which share strong non-Gaussian entanglement. We also discuss practical dissipation and dephasing effects on the cat state. The results indicate that strong nonclassicality and non-Gaussian entanglement are present in the two-mode cat state using fully feasible parameters.
The cavity-mediated coupling between magnons in an yttrium-iron-garnet (YIG) sphere and a superconducting qubit has recently been demonstrated as a new platform for preparing macroscopic quantum states. Here, based on this system, we propose to entangle two magnon modes in two YIG spheres by driving the qubit with a two-tone field and by appropriately choosing the frequencies and strengths of the two driving fields. We show that strong entanglement can be achieved with fully feasible parameters. We further provide a detection scheme for experimentally verifying the entanglement. Our results indicate that macroscopic entanglement between two magnon modes in two millimeter-sized YIG spheres, involving more than 101810^{18}1018 spins, can be realized using currently available parameters, which finds promising applications in fundamental studies, such as macroscopic quantum mechanics and the test of unconventional decoherence theories.
We propose a scheme for preparing magnon squeezed states in a hybrid cavity-magnon-qubit system. The system consists of a microwave cavity that simultaneously couples to a magnon mode of a macroscopic yttrium-iron-garnet sphere via the magnetic-dipole interaction and to a transmon-type superconducting qubit via the electric-dipole interaction. By far detuning from the magnon-qubit system, the microwave cavity is adiabatically eliminated. The magnon mode and the qubit then get effectively coupled via the mediation of virtual photons of the microwave cavity. We show that by driving the qubit with two microwave fields and by appropriately choosing the drive frequencies and strengths, magnonic parametric amplification can be realized, which leads to magnon quadrature squeezing with the noise below vacuum fluctuation. We provide optimal conditions for achieving magnon squeezing, and moderate squeezing can be obtained using currently available parameters. The generated squeezed states are of a magnon mode involving more than 1018{10}^{18}1018 spins and thus macroscopic quantum states. This work may find promising applications in quantum information processing and high-precision measurements based on magnons and in the study of macroscopic quantum states.
We propose a scheme to generate entanglement between magnon and superconducting qubit. The macroscopic yttrium–iron–garnet sphere and superconducting qubit are installed in two spatially separated cavities, which are directly driven by a two-mode squeezed-vacuum microwave field. The magnon and cavity 1 are coupled via magnetic dipole interaction and the superconducting qubit and cavity 2 are coupled via electric dipole interaction. We theoretically demonstrate that the magnon–qubit steady-state entanglement can be created by transferring quantum correlations of the two-mode squeezed-vacuum driving field via cavity–magnon and cavity–qubit beam-splitter interactions. The transfer is highly efficient, and the entanglement is robust against temperature in the optimal parameter regimes. We also deduce a new, to the best of our knowledge, mathematical method to analyze the dynamics of the magnon–qubit entanglement and some significant results are obtained. Our scheme can be implemented with experimentally feasible parameters and may provide guidance in designing hybrid quantum networks.
We study the hybrid system of a high-quality yttrium-iron-garnet (YIG) sphere magnetically coupled to a superconducting flux qubit via the magnetic fields created by two persistent-current quantum states. When the flux qubit is operated away from its sweet point, a coherent nonlinear two-magnon interaction can be predicted; that is, the qubit is excited by absorbing a pair of magnons. We show that the spontaneous emission of the qubit can be exploited to steer the magnon mode of the YIG sphere into a Schr\"odinger cat state with high fidelity. Our scheme has practical advantages in that it eliminates the need for precise control of the evolution time and the projective measurement, and is also insensitive to the qubit's pure dephasing.
We present protocols to generate quantum entanglement on nonlocal magnons in hybrid systems composed of yttrium iron garnet (YIG) spheres, microwave cavities and a superconducting (SC) qubit. In the schemes, the YIGs are coupled to respective microwave cavities in resonant way, and the SC qubit is placed at the center of the cavities, which interacts with the cavities simultaneously. By exchanging the virtual photon, the cavities can indirectly interact in the far-detuning regime. Detailed protocols are presented to establish entanglement for two, three and arbitrary N magnons with reasonable fidelities.
We theoretically propose a magnon-based superconducting qubit hybrid system containing two ferromagnetic yttrium iron garnets (YIGs) and one superconducting qubit, demonstrating that this hybrid quantum system is feasible in achieving the unconventional two-mode magnon blockade and generating the quantum correlated magnon pairs. Through the virtual-photon excitation mediated by the same cavity mode, the effective couplings can be established not only between the Kittel modes and the qubit, but also between the two Kittel modes. We characterize the magnon blockade by utilizing the equal-time second-order correlation functions and impose a constraint on the correlations between the two Kittel modes by introducing the Cauchy-Schwarz inequality (CSI). In the weak qubit-magnon coupling regime, our results indicate the following: (i) Under the scenario of the resonance couplings, the ratio of driving strengths and the ratio of coupling strengths between the two Kittel modes and the cavity mode can be employed to control and regulate the magnon blockade and the violation of the CSI. Not only the two Kittel modes can achieve the unconventional magnon antibunching effects simultaneously, but also the correlations between them violate the classical CSI. It is important to note that when the degrees of both the antibunching effects for the two Kittel modes and the anticorrelation between them reach their highest levels simultaneously, the correlations happen not to violate the CSI. But our proposal offers a feasible route for preparing a high-quality single-magnon source. (ii) Under the scenario of the out-of-resonance couplings, by adjusting the frequency detunings between the first Kittel mode and the second Kittel mode as well as the qubit, the quantum correlated two-mode magnon pairs can be obtained. (iii) Finally, it is found that the magnon antibunching effect of the second Kittel mode is robust against the dissipation ratio between the two Kittel modes. These results illustrate how to optimally choose the parameters to realize nonclassical magnon pair generation and CSI violation. The hybrid YIG quantum system we propose here holds the potential in the development of efficient microwave single-magnon sources and quantum magnon-magnon pairs required in future quantum information processing tasks.