The direct coupling system between a magnon and a superconducting qubit exhibits a special advantage in the quantum information processing [Phys. Rev. Lett. 129, 037205 (2022) ]. In this paper, based on this direct coupling system, the magnonic squeezed Schr\"odinger cat states (MSSCSs) can be generated by the conditional displacement mechanism. Numerical simulation shows that our scheme is robust to the dissipation of the system. In particular, we find that the coherence amplitude of MSSCSs can be enhanced via a squeezed transformation. Our results have potential application in quantum information processing and quantum optics.
The direct coupling system between a magnon and a superconducting qubit exhibits a special advantage in the quantum information processing [Phys. Rev. Lett. 129, 037205 (2022) ]. In this paper, based on this direct coupling system, the magnonic squeezed Schr"odinger cat states (MSSCSs) can be generated by the conditional displacement mechanism. Numerical simulation shows that our scheme is robust to the dissipation of the system. In particular, we find that the coherence amplitude of MSSCSs can be enhanced via a squeezed transformation. Our results have potential application in quantum information processing and quantum optics.
The hybrid system between the transmon and the magnon plays an important role in quantum information science. Additionally, the cat states of the magnon are a valuable quantum resource and represent macroscopic quantum superpositions of large numbers of spins. In this paper, we propose an approach to generate magnonic Schr\"odinger cat states (SCSs) in a hybrid ferromagnet-superconductor system. Moreover, high-fidelity magnonic SCSs can be generated when the dissipation of the system is considered. The coherent amplitude and the fidelities of the cat states can be adjusted by controlling the parameters of the system.
Hybrid systems based on magnons in ferromagnetic materials, such as yttrium iron garnet, have achieved remarkable development in the last decade. These include the coupling of magnons to microwave and optical photons, superconducting qubits, phonons, spins, the center-of-mass motion of a ferromagnet, etc. Here, we review both the experimental and theoretical progress in this field, focusing on the generation of magnonic quantum states and their applications in a broad range of fields. Since the strong coupling is a prerequisite for achieving coherent quantum control of magnons and preparing magnonic quantum states, we start by introducing representative strong-coupling experiments in cavity magnonics, then review a series of protocols for creating various magnonic quantum states, such as Fock, cat, squeezed, and entangled states, and discuss their potential applications in macroscopic quantum studies, quantum information science, quantum sensing, magnonic quantum devices, dark matter detection, and so on. Finally, we summarize the review and give an outlook for the future study of quantum magnonics.
We propose an efficient method for dissipative generation of magnonic cat states in a cavity-magnon-qubit hybrid system by exploiting a two-magnon driving and dissipation mechanism. When both the magnon and qubit are driven, a coherent nonlinear two-magnon interaction is induced, wherein the qubit and the magnon mode exchange energy through magnon pairs. The dissipation of the qubit is exploited to steer the magnon mode into a quantum superposition of distinct coherent states, where the magnon mode evolves into either an even or odd cat state, depending on the parity of the magnon initial state. For the case where the magnon initial state is a superposition state, e.g., of $|0\ensuremath{\rangle}$ and $|1\ensuremath{\rangle}$, the magnon mode can evolve into a weighted mixture of the even and odd cat states. We also find that magnon squeezed states may emerge during the short-time evolution, showcasing the capability of our mechanism in preparing diverse magnon nonclassical states. Magnonic cat and squeezed states are macroscopic quantum states and have applications in macroscopic quantum studies and quantum sensing, e.g., in the dark-matter search using ferromagnetic axion haloscopes.
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.
Quantum states are important resources and their preparations are essential prerequisites to all quantum technologies. However, they are extremely fragile due to the inevitable dissipations. Here the all-optical generation of a deterministic squeezed Schr\"odinger-cat state based on dissipation is proposed. Our system is based on the Fredkin-type interaction between three optical modes, one of which is subject to coherent two-photon driving and the others are coherent driving. We show that an effective degenerate three-wave-mixing process can be engineered in our system, which can cause the simultaneous loss of two photons, resulting in the generation of a deterministic squeezed Schr\"odinger-cat state. More importantly, by controlling the driving fields in our system, the two-photon loss can be adjustable, which can accelerate the generation of squeezed Schr\"odinger-cat states. In addition, we exploit the squeezed Schr\"odinger-cat states to estimate the phase in the optical interferometer and show that the quantum Fisher information about the phase can reach the Heisenberg limit in the limit of a large photon number. Meanwhile, it can have an order of magnitude factor improvement over the Heisenberg limit in the low-photon-number regime, which is very valuable for fragile systems that cannot withstand large photon fluxes. This work proposes an all-optical scheme to deterministically prepare the squeezed Schr\"odinger-cat state with high speed and can also be generalized to other physical platforms.
Abstract We propose a scheme to generate squeezed states in a cross-shaped cavity magnonics system by Kerr effect originating from the magnetocrystalline anisotropy. The system consists of two microwave cavities and a yttrium iron garnet sphere supporting a magnon mode embodying the collective spin motion, and each cavity mode is coupled with the magnon mode through the magnetic-dipole interaction. It is demonstrated that simultaneous squeezing of the two microwave cavity modes and the magnon mode can be achieved due to Kerr nonlinearity, and the squeezing of the magnon mode can break the 3 dB limit. By utilizing a Bogoliubov transformation, a new perspective on the underlying physical mechanism for the generation of the squeezing using Kerr nonlinearity is provided. Moreover, we also demonstrate the strong robustness of the magnon mode’s squeezing against the system dissipation and environmental temperature. The present scheme provides a promising platform for quantum information processing, holding significant implications for quantum metrology and precision measurement.