Ultrastrong light-matter coupling opens exciting possibilities to generate squeezed quantum states and entanglement. We propose achieving this regime in superconducting hybrid nanostructures with ferromagnetic interlayers. Strong confinement of the electromagnetic field between superconducting plates results in the existence of magnon-polariton (MP) modes with ultrastrong magnon-photon coupling, ultrahigh cooperativity, and colossal group velocities. These modes provide a numerically accurate explanation of recent experiments and have intriguing quantum properties. The MP quantum vacuum consists of the squeezed magnon and photon states with the degree of squeezing controlled in wide limits by the external magnetic field. The ground-state population of virtual photons and magnons is vast and can be used for generating correlated magnon and photon pairs. MP excitations contain bipartite entanglement between magnons and photons. Our results indicate that superconducting/ferromagnet nanostructures are very promising for quantum magnonics.
Ultrastrong light-matter coupling opens exciting possibilities to generate squeezed quantum states and entanglement. We propose achieving this regime in superconducting hybrid nanostructures with ferromagnetic interlayers. Strong confinement of the electromagnetic field between superconducting plates results in the existence of magnon-polariton (MP) modes with ultrastrong magnon-photon coupling, ultrahigh cooperativity, and colossal group velocities. These modes provide a numerically accurate explanation of recent experiments and have intriguing quantum properties. The MP quantum vacuum consists of the squeezed magnon and photon states with the degree of squeezing controlled in wide limits by the external magnetic field. The ground-state population of virtual photons and magnons is vast and can be used for generating correlated magnon and photon pairs. MP excitations contain bipartite entanglement between magnons and photons. Our results indicate that superconducting/ferromagnet nanostructures are very promising for quantum magnonics.
Rigidity of an ordered phase in condensed matter results in collective excitation modes spatially extending in macroscopic dimensions. Magnon is a quantum of an elementary excitation in the ordered spin system, such as ferromagnet. Being low dissipative, dynamics of magnons in ferromagnetic insulators has been extensively studied and widely applied for decades in the contexts of ferromagnetic resonance, and more recently of Bose-Einstein condensation as well as spintronics. Moreover, towards hybrid systems for quantum memories and transducers, coupling of magnons and microwave photons in a resonator have been investigated. However, quantum-state manipulation at the single-magnon level has remained elusive because of the lack of anharmonic element in the system. Here we demonstrate coherent coupling between a magnon excitation in a millimetre-sized ferromagnetic sphere and a superconducting qubit, where the interaction is mediated by the virtual photon excitation in a microwave cavity. We obtain the coupling strength far exceeding the damping rates, thus bringing the hybrid system into the strong coupling regime. Furthermore, we find a tunable magnon-qubit coupling scheme utilising a parametric drive with a microwave. Our approach provides a versatile tool for quantum control and measurement of the magnon excitations and thus opens a new discipline of quantum magnonics.
Quantum electrodynamics lies at the heart of hybrid quantum systems essential for future technologies. The thermodynamic limit of the Dicke model, a fundamental model describing these systems, predicts exotic quantum phenomena, such as equilibrium superradiant phase transitions and ground-state two-mode squeezing. However, the experimental realization of genuine Dicke systems has remained elusive due to the inevitable existence of gauge-invariant self-interaction terms that hinder such phenomena. Here, we report on the on-chip realization of a Dicke-type system utilizing ultrastrong magnetic-dipole interactions between collective excitations in a spatially separated ferromagnetic array and a superconducting resonator, resulting in creation of magnon polaritons. Crucially, this spatially separated architecture allows the cooperative enhancement of the coupling strength without increasing the self-interaction energy. We experimentally confirm the Bloch-Siegert shift, originating from the counter-rotating terms, alongside the suppression of self-excitation terms required to observe critical Dicke physics. Our results establish a versatile platform, which provides the playground to explore quantum collective coupling physics and open pathways towards integrated quantum devices harnessing Dicke physics.
Dissipation usually has negative effects on magnon-photon entanglement and thus is engineered to a low level in many works. In this work, we propose a theoretical scheme to generate steady-state magnon-photon entanglement by using the dissipatively coupled magnon and photon in a squeezed vacuum reservoir. It is found that the reservoir-mediated magnon-photon coupling and the squeezing nature of reservoir result in the entanglement. As the coupling strength increases or intrinsic dissipation decreases, the magnon-photon correlation and entanglement can be enhanced. As the squeezing amplitude r of reservoir increases, the logarithmic negativity, which is the measure of entanglement, presents a maximum at an optimal value of r. This reflects the competition between the enhancement of magnon-photon correlation which favors the entanglement, and the increase in the input noise which destroys the entanglement. We also propose a device geometry which could be realized with existing technology of cavity magnon polariton and superconducting Josephson parametric amplifiers. Our theoretical methods and results may be useful for future theoretical studies of magnon-photon or magnon-magnon entanglement and experimental studies of magnon-based quantum communication and manipulations.
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.
The techniques of microwave quantum optics are applied to collective spin excitations in a macroscopic sphere of a ferromagnetic insulator. We demonstrate, in the single-magnon limit, strong coupling between a magnetostatic mode in the sphere and a microwave cavity mode. Moreover, we introduce a superconducting qubit in the cavity and couple the qubit with the magnon excitation via the virtual photon excitation. We observe the magnon–vacuum-induced Rabi splitting. The hybrid quantum system enables generation and characterization of non-classical quantum states of magnons.
We investigate the second-order photon correlation function in cavity-magnon systems, focusing on ferromagnetic and antiferromagnetic cavities within the ultrastrong coupling regime, and extending beyond the rotating-wave approximation. By deriving exact integral solutions for the second-order correlation function, we demonstrate that counter-rotating magnon-photon interactions induce quadrature squeezing in the cavity mode. Furthermore, we show that tuning the anisotropic magnon-cavity couplings enhances the squeezing effect by changing the level repulsion of the magnon-cavity photon hybrid mode without increasing the cavity photon occupation number. Our study reveals distinct quantum correlation behaviors in ferromagnetic and antiferromagnetic cavities: For ferromagnetic cavities, we show that squeezing increases with coupling strength asymmetry, whereas in the antiferromagnetic case, magnon modes with opposite chirality suppress quantum effects and impose a lower bound on correlation functions. These findings provide a pathway to optimize photon blockade for quantum information technology in magnon-cavity systems in the ultrastrong coupling limit.