Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2507.06065

On-chip Dicke-type magnon polaritons in the ultrastrong coupling regime via spatially separated nanomagnets

Shugo Yoshii, Manuel Müller, Ryo Ohshima, Matthias Althammer, Yuichiro Ando, Hans Huebl, Masashi Shiraishi

Published Jul 8, 2025• 0 citations
arXivHTMLPDF

Abstract

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.

DOI
10.48550/arxiv.2507.06065

Related papers

Paramagnets as a platform for Dicke physics

T. Elijah Kritzell, Jacques Doumani, Tobias Asano +11 more

The interaction of an ensemble of two-level atoms and a quantized electromagnetic field, described by the Dicke Hamiltonian, is an extensively studied problem in quantum optics. However, experimental efforts to explore similar physics in condensed matter typically employ bosonic matter modes (e.g., phonons, magnons, and plasmons) that are describable as simple harmonic oscillators, i.e., an infinite ladder of equally spaced energy levels. Here, we examine ultrastrong coupling between a coherent light mode and an ensemble of paramagnetic spins, a finite-multilevel system, in Gd3Ga5O12. The electron paramagnetic resonance of Gd3+ ions is tuned by a magnetic field into resonance with a Fabry–Pérot cavity mode, resulting in the formation of spin–photon hybrid states, or Zeeman polaritons. We observe that the light–matter coupling strength, measured through the vacuum Rabi splitting, decreases with increasing temperature, which can be explained by the temperature-dependent population difference between the lower and higher-energy states, a trait of a finite-level system. This finding demonstrates that a spin–boson system is more compatible with the Dicke model and has advantages over boson–boson systems for pursuing experimental realizations of phenomena predicted for ultrastrongly coupled light–matter hybrids. Realizing ultrastrong light-matter coupling in condensed matter systems has largely relied on bosonic matter modes. Here, the authors report ultrastrong coupling between a cavity photon mode and paramagnetic spins in gadolinium gallium garnet, consistent with the spin-boson physics described by the Dicke model.

Nature Communications•Aug 5, 2026•0 citations

Zeeman polaritons as a platform for probing Dicke physics in condensed matter

T. Elijah Kritzell, Jacques Doumani, Asano, Tobias +11 more

The interaction of an ensemble of two-level atoms and a quantized electromagnetic field, described by the Dicke Hamiltonian, is an extensively studied problem in quantum optics. However, experimental efforts to explore similar physics in condensed matter typically employ bosonic matter modes (e.g., phonons, magnons, and plasmons) that are describable as simple harmonic oscillators, i.e., an infinite ladder of equally spaced energy levels. Here, we examine ultrastrong coupling between a coherent light mode and an ensemble of paramagnetic spins, a finite-multilevel system, in Gd$_3$Ga$_5$O$_{12}$. The electron paramagnetic resonance of Gd$^{3+}$ ions is tuned by a magnetic field into resonance with a Fabry--Pérot cavity mode, resulting in the formation of spin--photon hybrid states, or Zeeman polaritons. We observe that the light--matter coupling strength, measured through the vacuum Rabi splitting, decreases with increasing temperature, which can be explained by the temperature-dependent population difference between the lower and higher-energy states, a trait of a finite-level system. This finding demonstrates that a spin--boson system is more compatible with the Dicke model and has advantages over boson--boson systems for pursuing experimental realizations of phenomena predicted for ultrastrongly coupled light--matter hybrids.

arXiv:2409.17339•Sep 25, 2024•1 citation

Ultrastrong magnon-photon coupling, squeezed vacuum, and entanglement in superconductor/ferromagnet nanostructures

М. А. Силаев

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.

arXiv:2211.00462•Physical review. B./Physical review. B•May 26, 2023•22 citations

Strong Coupling between Microwave Photons and Nanomagnet Magnons

Justin T. Hou, Luqiao Liu

Coupled microwave photon-magnon hybrid systems offer promising applications by harnessing various magnon physics. At present, in order to realize high coupling strength between the two subsystems, bulky ferromagnets with large spin numbers are utilized, which limits their potential applications for scalable quantum information processing. By enhancing single spin coupling strength using lithographically defined superconducting resonators, we report high cooperativities between a resonator mode and a Kittel mode in nanometer thick Permalloy wires. The on-chip, lithographically scalable, and superconducting quantum circuit compatible design provides a direct route towards realizing hybrid quantum systems with nanomagnets, whose coupling strength can be precisely engineered and dynamic properties can be controlled by various mechanisms derived from spintronic studies.

arXiv:1903.01887•Physical Review Letters•Sep 3, 2019•266 citations

Coherent coupling between ferromagnetic magnon and superconducting qubit

Yutaka Tabuchi, Seiichiro Ishino, Atsushi Noguchi +4 more

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.

arXiv:1410.3781•Oct 14, 2014•5 citations

Implementation of the Dicke Lattice Model in Hybrid Quantum System Arrays

Liujun Zou, D. Marcos, Sebastian Diehl +4 more

Generalized Dicke models can be implemented in hybrid quantum systems built from ensembles of nitrogen-vacancy (NV) centers in diamond coupled to superconducting microwave cavities. By engineering cavity assisted Raman transitions between two spin states of the NV defect, a fully tunable model for collective light-matter interactions in the ultrastrong coupling limit can be obtained. Our analysis of the resulting nonequilibrium phases for a single cavity and for coupled cavity arrays shows that different superradiant phase transitions can be observed using existing experimental technologies, even in the presence of large inhomogeneous broadening of the spin ensemble. The phase diagram of the Dicke lattice model displays distinct features induced by dissipation, which can serve as a genuine experimental signature for phase transitions in driven open quantum systems.

arXiv:1405.3289•Physical Review Letters•Jul 10, 2014•122 citations
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.
The interaction of an ensemble of two-level atoms and a quantized electromagnetic field, described by the Dicke Hamiltonian, is an extensively studied problem in quantum optics. However, experimental efforts to explore similar physics in condensed matter typically employ bosonic matter modes (e.g., phonons, magnons, and plasmons) that are describable as simple harmonic oscillators, i.e., an infinite ladder of equally spaced energy levels. Here, we examine ultrastrong coupling between a coherent light mode and an ensemble of paramagnetic spins, a finite-multilevel system, in Gd3Ga5O12. The electron paramagnetic resonance of Gd3+ ions is tuned by a magnetic field into resonance with a Fabry–Pérot cavity mode, resulting in the formation of spin–photon hybrid states, or Zeeman polaritons. We observe that the light–matter coupling strength, measured through the vacuum Rabi splitting, decreases with increasing temperature, which can be explained by the temperature-dependent population difference between the lower and higher-energy states, a trait of a finite-level system. This finding demonstrates that a spin–boson system is more compatible with the Dicke model and has advantages over boson–boson systems for pursuing experimental realizations of phenomena predicted for ultrastrongly coupled light–matter hybrids. Realizing ultrastrong light-matter coupling in condensed matter systems has largely relied on bosonic matter modes. Here, the authors report ultrastrong coupling between a cavity photon mode and paramagnetic spins in gadolinium gallium garnet, consistent with the spin-boson physics described by the Dicke model.
The interaction of an ensemble of two-level atoms and a quantized electromagnetic field, described by the Dicke Hamiltonian, is an extensively studied problem in quantum optics. However, experimental efforts to explore similar physics in condensed matter typically employ bosonic matter modes (e.g., phonons, magnons, and plasmons) that are describable as simple harmonic oscillators, i.e., an infinite ladder of equally spaced energy levels. Here, we examine ultrastrong coupling between a coherent light mode and an ensemble of paramagnetic spins, a finite-multilevel system, in Gd3_33​Ga5_55​O12_{12}12​. The electron paramagnetic resonance of Gd3+^{3+}3+ ions is tuned by a magnetic field into resonance with a Fabry--Pérot cavity mode, resulting in the formation of spin--photon hybrid states, or Zeeman polaritons. We observe that the light--matter coupling strength, measured through the vacuum Rabi splitting, decreases with increasing temperature, which can be explained by the temperature-dependent population difference between the lower and higher-energy states, a trait of a finite-level system. This finding demonstrates that a spin--boson system is more compatible with the Dicke model and has advantages over boson--boson systems for pursuing experimental realizations of phenomena predicted for ultrastrongly coupled light--matter hybrids.
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.
Coupled microwave photon-magnon hybrid systems offer promising applications by harnessing various magnon physics. At present, in order to realize high coupling strength between the two subsystems, bulky ferromagnets with large spin numbers are utilized, which limits their potential applications for scalable quantum information processing. By enhancing single spin coupling strength using lithographically defined superconducting resonators, we report high cooperativities between a resonator mode and a Kittel mode in nanometer thick Permalloy wires. The on-chip, lithographically scalable, and superconducting quantum circuit compatible design provides a direct route towards realizing hybrid quantum systems with nanomagnets, whose coupling strength can be precisely engineered and dynamic properties can be controlled by various mechanisms derived from spintronic studies.
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.
Generalized Dicke models can be implemented in hybrid quantum systems built from ensembles of nitrogen-vacancy (NV) centers in diamond coupled to superconducting microwave cavities. By engineering cavity assisted Raman transitions between two spin states of the NV defect, a fully tunable model for collective light-matter interactions in the ultrastrong coupling limit can be obtained. Our analysis of the resulting nonequilibrium phases for a single cavity and for coupled cavity arrays shows that different superradiant phase transitions can be observed using existing experimental technologies, even in the presence of large inhomogeneous broadening of the spin ensemble. The phase diagram of the Dicke lattice model displays distinct features induced by dissipation, which can serve as a genuine experimental signature for phase transitions in driven open quantum systems.